Modal Verbs

Can / can’t  and could / couldn’t for factual (im)possibility

• We use can / can’t to say what is generally possible / impossible in fact.
  • It can get very hot at this time of year.
  • Plants can’t survive without sunlight.
• We use could / couldn’t to say what was / wasn’t generally possible in fact in the past.
  • Before planes were invented it could take three months to travel from Europe to Australia.
  • Women couldn’t vote in Britain before 1918.
• We sometimes use How could I / you? when we are angry with our mistake or with someone’s behavior.
• How could I have been so stupid?

Will / won’t for factual certainty and habitual behavior

• Will / won’t are not always about the future. We can use will / won’t to refer to habitual present behavior.
  • They’ll always leave a clue in the story.
  • Jake’s so lazy – often he won’t leave his house all weekend!
• We use will / won’t when we are certain that something is always true.
  • At this time of the year there’ll be a storm nearly every afternoon.

Will / won’t, will / won’t have + past participle; will / won’t be + -ing for assumptions

• We sometimes use will / won’t or will / won’t be + -ing to assume things about now, and will have + past participle to assume things about the past.
  • No, it won’t be that. (= I’m sure it isn’t that.)
  • George’ll be playing tennis – he always plays on Sunday afternoons.
  • The receptionist will have kept the money.

Might (not), may (not), could(n’t) and can’t for theoretical (im)possibility

• We use might (not), may (not) or could to say we think something is possible now or in the future.
  • He may be too busy.
  • He might not be busy. NOT He could not be busy. 
• We sometimes add well (to emphasise a possibility) or just (to emphasise that although unlikely, something is still possible) between might, may or could and the infinitive.
  • Tom thinks it’s going to rain, and he may well be right.
  • I know it sounds like a stupid idea, but it might just work.
• When we ask questions about theoretical possibility now or in the future we use could / might (not may).
  • Could it be something to do with her? NOT May it be …?
• We use can’t / couldn’t to say we think something is impossible.
  • That can’t be right.

Might (not) have, may (not) have, could have and can’t have + past participle for theoretical possibility in the past

• We use might (not) / may (not) have or could have + past participle, to say we think a past event / situation was possible.
  • One of the friends might have found it on the floor.
• We usually prefer may in more formal situations and when we are a little more confident about something.
  • You may know that Matt and I were very good friends. I may come with you if I have time. I might come, but it’s not very likely.
• We use might / could have + past participle to ask if something was theoretically possible in the past.
  • Could she have taken the money?
• We use can’t / couldn’t have + past participle to say we think a past event or event was impossible.
  • He can’t have kept more than $ 20.
  • We couldn’t have got here any earlier.

Must and must have + past participle for theoretical certainty.

• We use must (or have [got] to in more informal situations when we have reason to believe something is true in the present.
  • There must be something else.
  • There has to be a simple answer.
• We use must have + past participle when we have reason to believe something happened / was true in the past.
  • The receptionist must have kept the money.

Source: Active Grammar, Level 3, Mark Lloyd and Jeremy Day, CUP, 2011 (With Ramrowriter’s adaptation where necessary)