What are adverbs of possibility?

They are words such as certainly, definitely, probably, possibly, perhaps, and maybe that tell you how sure the speaker is about a statement. They do not change the tense of the verb. They add a comment, usually in mid position (She will probably call) or at the front of the clause (Perhaps she will call).

Where do I put probably in a sentence?

After the first auxiliary or after be: She has probably left. They are probably stuck in traffic. If there is no auxiliary, put it before the main verb: She probably left. Do not place it between the verb and the object, and do not use it in front of a noun.

What is the difference between maybe and may be?

Maybe is one word and an adverb, usually at the start of a sentence: Maybe she's at the library. May be is two words, the modal may plus the verb be: She may be at the library. The meanings are close; the grammar is not.

Should I use perhaps or maybe?

Perhaps is slightly more formal and is at home in writing and in polite suggestions: Perhaps we could postpone. Maybe is the default in conversation: Maybe we could postpone. Both can be a one-word answer. Neither belongs in mid position as comfortably as probably does.

Are these the same as modals like might and could?

No. Might and could are auxiliary verbs: they invert in questions, take not, and are followed by a bare infinitive. Adverbs of possibility are comments on a clause whose verb is already chosen. You can combine them: She might possibly agree. You cannot replace a modal's grammar with an adverb's position, or the other way round.

English grammar · Intermediate

Adverbs of Possibility: Certainly, Probably, Perhaps, and Maybe

Written by Eleanor Whitfield, Lead English Curriculum Editor · Reviewed September 2026

Adverbs of possibility tell you how likely the speaker thinks a statement is: certainly, definitely, probably, possibly, perhaps, maybe. They do not change the grammar of the verb the way a modal does; they add a comment on the whole clause and sit in a small set of positions, most often mid-clause after the first auxiliary or before a simple verb. This page is about those adverbs and their placement. Modals of possibility (might, may, could) and intensifiers (very, absolutely) are neighbouring systems, not the same one.

She'll probably be late; the traffic is usually heavy on Fridays.

probably sits in mid position and grades the likelihood of being late

What it is

English can mark how sure a speaker is without changing the verb at all. An adverb of possibility sits in the clause and grades the statement from almost certain to merely possible: She'll certainly agree. She'll probably agree. She'll possibly agree. Perhaps she'll agree. The verb stays in whatever tense the facts need; the adverb is extra commentary. That is the difference from a modal, which occupies the finite slot (She might agree) and from an intensifier, which grades an adjective (very likely) rather than the whole proposition. Position is the grammar you actually have to learn, because certainly and probably prefer mid position, while perhaps and maybe prefer the front of the clause.

They have definitely missed the train. (definitely after have, high certainty) Maybe they missed the train. (maybe at the front, a looser suggestion)

How to spot it

Look for a word that answers how sure are you, not how or when. If it can move to the front of the sentence and still comment on the whole idea (Probably she left. Perhaps she left.), it is in this set. If it has to sit next to an adjective (very tired, absolutely freezing), it is an intensifier instead.

I will certainly be there by six. (certainly after will: high certainty)
They probably took the longer route. (probably before a simple past verb)
She could possibly join us later. (possibly after a modal, a weaker chance)
Perhaps the shop is shut on Mondays. (perhaps at the front of the clause)

Likely is an adjective in careful English (a likely result, She is likely to agree) and only a borderline adverb in informal speech (She'll likely agree). Prefer probably for the adverb slot.

Adverbs of Possibility Quick Reference

The scale and the preferred position

Adverb	How sure	Typical position
certainly / definitely	very high: the speaker treats it as settled	mid: after the first auxiliary, or before a simple verb
probably	high, but not locked	mid: the same slot as certainly
possibly	real but weak	mid, or after a modal: could possibly
perhaps	open: a suggestion rather than a calculation	front of the clause, especially in writing
maybe	open: the spoken counterpart of perhaps	front of the clause, especially in speech
likely (adjective)	high, but not an adverb in careful style	be likely to + infinitive: She is likely to agree

Common Adverbs of Possibility Examples in English

Grouped by how much commitment the speaker is making, because that is what these adverbs exist to do:

Treating It as Settled

The keys are certainly in the kitchen drawer.: certainly after are
She has definitely read the brief; her notes are all over it.: definitely after has
I certainly don't want another meeting tonight.: certainly before don't, grading a negative
They will definitely have finished by the time we arrive.: definitely after will, before have

Certainly and definitely sit after the first auxiliary (is, has, will, can) and before not when the auxiliary is contracted onto the verb: I certainly don't, not I don't certainly in careful style.

A Strong but Open Prediction

He'll probably take the later flight.: probably after will
The restaurant is probably fully booked on Saturday.: probably after is
They probably haven't seen the announcement yet.: probably before haven't
I'd probably refuse if they asked me tomorrow.: probably after would

Probably is the workhorse of this set. It is the default way to mark a likely future or a likely explanation without the heavier commitment of certainly.

A Real but Weak Chance

She might possibly change her mind.: possibly after might, stacking two weak markers
The parcel has possibly been delivered to the wrong block.: possibly after has, a tentative explanation
We could possibly squeeze in one more chair.: could possibly: a cautious offer
It will possibly rain on the west side of the hills.: possibly after will, a low-confidence forecast

Possibly often partners a modal (might, could, may) because both mark a weak chance. Using both together is common and not considered a double mark in the way very unique is.

Suggestions from the Front of the Clause

Perhaps we should wait until the rain eases.: perhaps at the front, a polite suggestion
Maybe the battery is just loose.: maybe at the front, a spoken hypothesis
Perhaps she didn't get the invitation.: perhaps grading a past negative
Maybe next week would be better.: maybe plus a whole clause as a proposal

Front position is the natural home of perhaps and maybe. Mid position is possible (She perhaps left early) but sounds stiffer, and maybe in mid position is rare.

How Adverbs of Possibility Work

They comment on the clause. Position does most of the grammar; the scale does most of the meaning.

Mid Position After the First Auxiliary

Certainly, definitely, probably, and possibly prefer to sit after the first auxiliary or after be, and before the main verb: She will probably call. They have certainly left. With a simple tense and no auxiliary, they sit in front of the verb: She probably left. They do not normally sit between a verb and its object: She left probably the house is ungrammatical.

We have probably missed the opening remarks.

probably after have, before missed

After the first auxiliary, or before a simple verb.

Front Position for Perhaps and Maybe

Perhaps and maybe comment on the whole sentence and are happiest at the start: Perhaps she's stuck on the ring road. Maybe we should text her. They can also stand as a one-word reply. Moving probably to the front (Probably she's stuck) is possible in speech but is less standard than mid position.

Perhaps the museum closes at four on Tuesdays.

perhaps at the front, commenting on the whole clause

Perhaps and maybe lead the sentence.

They Grade the Clause, Not the Adjective

An intensifier grades a quality: very late, absolutely certain. An adverb of possibility grades the speaker's commitment to the statement: She is probably late. Stacking the two is fine because they target different things: She is probably very late. What you cannot do is treat probably as if it were very (a probably late train) or treat very as if it were probably (She very is late).

He is probably extremely tired after the shift.

probably grades the statement; extremely grades tired

Possibility adverbs grade the sentence; intensifiers grade the word.

They Combine with Modals, They Do Not Replace Them

A modal occupies the finite slot and can carry other meanings (ability, permission) as well as possibility. An adverb of possibility can sit alongside a modal to fine-tune it: She might possibly agree (weaker than She might agree), She will certainly agree (stronger than She will agree). You do not drop the modal just because an adverb is present, and you do not drop the adverb's mid-position rule just because a modal is there.

They could probably finish the draft tonight.

could carries ability-or-possibility; probably grades how likely the finishing is

Modal for the verb slot; adverb for the comment.

Common Mistakes with Adverbs of Possibility

Incorrect: She left probably the office before five.

Correct: She probably left the office before five.

Probably cannot sit between the verb and its object. With a simple past and no auxiliary, the adverb goes in front of the verb, not after it and not in the middle of the verb phrase.

Incorrect: She maybe is at the library.

Correct: Maybe she is at the library. / She may be at the library.

Maybe prefers the front of the clause. Mid position with maybe is clumsy. If you want a mid-clause marker of possibility, switch to the two-word modal may be, or to probably / possibly after the auxiliary.

Incorrect: It's a probably mistake.

Correct: It's probably a mistake.

Probably is not an adjective and cannot sit in front of a noun the way likely can (a likely mistake). It has to comment on the clause, so it sits after is.

Incorrect: She will possible arrive later.

Correct: She will possibly arrive later. / She may arrive later.

Possible is the adjective (a possible delay). The adverb that grades a verb phrase is possibly. Mixing the two word classes is a common spelling-level error with this pair.

Perhaps vs. Maybe, and Short Answers

The two front-position adverbs are not full synonyms, and the whole set can stand alone as a reply.

Perhaps is slightly more formal; maybe is the spoken default

In writing, perhaps is at home in suggestions, hedges, and academic caution. In conversation, maybe is far more common. Both can open a clause, and both can be a one-word answer. Maybe in mid position (She maybe left) is informal and still marked; perhaps in mid position is possible but heavy.

Perhaps a smaller sample would be wiser.: written suggestion
Maybe we should order now, before the kitchen closes.: spoken suggestion
“Will she apply?” “Perhaps.”: one-word hedge

May be (two words) is the modal may plus be, and is not an adverb: She may be late. Maybe (one word) is the adverb: Maybe she's late.

Short answers and tags

The mid-position adverbs can sit in a short answer after the auxiliary: Will they come? They probably will. Has she left? She certainly has. Definitely and probably also work as stand-alone replies in speech, a little more committed than perhaps.

“Are you joining us?” “I probably will.”: probably plus a repeated auxiliary
“Is the printer working?” “It definitely isn't.”: definitely before a negative short answer
“Do you think it's genuine?” “Possibly.”: a one-word mid-scale reply

In a short answer the adverb sits before the auxiliary when the answer is negative (I probably won't, It certainly isn't) and either before or after a positive auxiliary (I probably will / I will, probably).

Quick Check: Adverbs of Possibility

Put the adverb in brackets into the most natural position, or choose the right word.

1. They have ___ missed the turning. (probably)

Answer: They have probably missed the turning.

mid position after have

2. ___ we should take the earlier train. (Perhaps / Possible)

Answer: Perhaps we should take the earlier train.

perhaps at the front; possible is an adjective

3. She ___ won't have time on Thursday. (certainly)

Answer: She certainly won't have time on Thursday.

certainly before a contracted negative

4. Rewrite as one word or two: She ___ late. (maybe / may be) meaning it is possible she is late, using a modal.

Answer: She may be late.

may be is the modal plus be

5. “Will the shop still be open?” “It ___ will.” (definitely)

Answer: It definitely will.

adverb plus a short-answer auxiliary

Adverbs of Possibility FAQs

What are adverbs of possibility?: They are words such as certainly, definitely, probably, possibly, perhaps, and maybe that tell you how sure the speaker is about a statement. They do not change the tense of the verb. They add a comment, usually in mid position (She will probably call) or at the front of the clause (Perhaps she will call).
Where do I put probably in a sentence?: After the first auxiliary or after be: She has probably left. They are probably stuck in traffic. If there is no auxiliary, put it before the main verb: She probably left. Do not place it between the verb and the object, and do not use it in front of a noun.
What is the difference between maybe and may be?: Maybe is one word and an adverb, usually at the start of a sentence: Maybe she's at the library. May be is two words, the modal may plus the verb be: She may be at the library. The meanings are close; the grammar is not.
Should I use perhaps or maybe?: Perhaps is slightly more formal and is at home in writing and in polite suggestions: Perhaps we could postpone. Maybe is the default in conversation: Maybe we could postpone. Both can be a one-word answer. Neither belongs in mid position as comfortably as probably does.
Are these the same as modals like might and could?: No. Might and could are auxiliary verbs: they invert in questions, take not, and are followed by a bare infinitive. Adverbs of possibility are comments on a clause whose verb is already chosen. You can combine them: She might possibly agree. You cannot replace a modal's grammar with an adverb's position, or the other way round.