Modal Verbs for Possibility

We can use modal verbs to talk about possibility or to express uncertainty in the present and future using modal verbs may, might and could.

Some suggest that ‘might‘ is less certain than ‘may‘, but in spoken English there is really no difference. However, ‘might‘ is more common in spoken English. The word ‘may‘ is less common, and we can only use ‘could‘ in the positive form, not the negative form, for talking about possibility.

Here’s how ‘may‘, ‘might‘ and ‘could‘ relate to other modal verbs of probability:

Thus, we use ‘may‘, ‘might‘ and ‘could‘ when we think that something is possible. Statements with ‘may’, ‘might’ and ‘could‘ can refer to past, present, or future possibilities.

Present possibility

 To refer to present possibility, use:

May / Might / Could + the verb (base form without ‘to’)

• Look at Jack! He might be in a hurry to meet with Catherine.
• The clouds cover the top of the mountain. It might be windy there.
• Don’t eat that mushroom. It could be poisonous.

Past possibility

Positive possibilities

When talking about past positive possibilities, use:

May / Might / Could + have + Past Participle

• I can’t find my pen. I might have dropped it earlier.
• The person who stole the money could have been one of the employees.
• He may have misunderstood you when you talked to him yesterday.
  

‘Could have‘ is usually used in unreal conditions – when we are imagining a possibility if something in the past had been different:

• If we had started this project earlier, we could have finished on time.

Negative possibilities

To refer to future possibility, use:

May / Might / Could + the verb (base form without ‘to’) + future time marker

• John’s not here. He might not have known about the meeting.
• If she hasn’t called you back, she may not have listened to your voicemail yet.

Note that ‘couldn’t have’ is only used when we are certain that something is impossible in the past:

• She couldn’t have taken the car; she doesn’t have a key.

Future possibility

To refer to future possibility, use:

May / Might / Could + the verb (base form without ‘to’) + future time marker

• It’s cold outside. It may snow later on.
• Joe might come to our party next weekend.
• Your daughter is really smart. She could be very successful someday.
  

Here’s a good video from Oxford Online English explaining how to use ‘can’, ‘may’, ‘might’ and ‘could’ to express possibility:

Read more on modal verbs:

Modal Verbs: Overview

Modal Verbs for Deduction

Modal Verbs for Obligations