May, Might, Can and Could: Possibility

May, Might, Can and Could: Possibility 1

In affirmative sentences (that is, sentences which are not questions or negatives), we use may or might to say there is a possibility of something happening or being true:
• This may/might be his last major speech before the election.
• The news may/might come as a shock to many of the people present.
• When Frank gets a job, I may/might get the money back that I lent him.
There is often little difference in meaning, but might can suggest that there is less possibility.
We can also use could, but not can, to express a similar meaning. We prefer could to show that we are giving an opinion about which we are unsure:
• 'Why isn't Tim here yet?' 'It may/might/could be because his mother is ill again.'
• There may/might/could be some cake left. I'll go and look.
We can use can in affirmative sentences when we talk about a more general possibility of something happening rather than the possibility of something happening in a particular situation:
• The temperature can sometimes reach 35°C in July.
• Mountain daisies can be yellow or red.
• It may/might/could rain later, {not It can...)
We prefer may rather than can in more formal contexts:
• Exceeding the stated dose may cause drowsiness, (from a medicine container)
We don't use may to ask questions about the possibility of something happening. Instead we use, for example, could(n't) or the phrase be likely:
• Could it be that you don't want to leave?
• Are you likely to be visiting Greece again this summer?
It is possible to use might in this type of question, but it is rather formal:
• Might they be persuaded to change their minds?
In negative sentences, including sentences with words like only, hardly or never, to say that something is not the case we can use can't (or more formally cannot) or couldn't (or could not):
• There can't/couldn't be any milk left - 1 would have seen it in the fridge.
• There can/could hardly be any doubt that he was guilty.
Compare the use of may/might and can/could in negative sentences:
• There are plans to rebuild the town centre, but it may not / might not happen for another ten years. (= It is possible that it won't happen for another ten years.)
• There are plans to rebuild the town centre, but it can't/couldn't happen for another ten years. (= It is not possible that it will happen for another ten years.)
The difference is that we use may not or might not to say that it is possible that something is not true, and can't or couldn't to say that it is not possible that something is true.
We use may well, might well or could well to say it is likely that something will happen:
• The profits of the company may/might/could well reach $100 million this year.
We don't use can well in this way to talk about the future. However, can well is used to talk about something we think or feel now:
• I can well recall how I felt when John told us he was moving to South Africa.
Other words commonly used after may, might, could and can to say it is possible that something will happen are conceivably and possibly:
• The President may conceivably call an election in June. (= it is possible to believe it)
• The new parking restrictions could possibly lead to fewer cars in our cities.

May, Might, Can and Could: Possibility 2

Compare these sentences:
• I'll write the date of the meeting in my diary, otherwise I may/might/could forget it. (= talking about present or future possibility)
• Jenny's late. She may/might/could have forgotten about the meeting. (= talking about past possibility)
We use may/might/could (not 'can') + have + past participle to say it is possible that something happened in the past:
• I thought I saw Tom in town, but I may/might could have been wrong.
• 'Where's Barbara's camera?' 'She may/might/could have taken it with her.'
We use might/could (not 'may' or 'can') + have + past participle to say that something was possible in the past, but we know that it did not in fact happen:
• If I hadn't come along at that moment, Jim might/could have been the one arrested instead of the real thief.
• The plan might/could easily have gone wrong, but in fact it was a great success.
We use might (not 'may') + infinitive to talk about what was typically the case in the past:
• During the war, the police might arrest you for criticising the king.
• Years ago children might be sent down mines at the age of six. (passive form)
We can also use could + infinitive in examples like this to talk about past ability. For example, 'During the war, the police could arrest you...' means that the police were legally able to arrest you.
We use may/might (not 'can') + have + past participle to say that by some time in the future, it is
possible that something will have happened:
• By next Friday I may/might have completed the report.
• His maths may/might have improved by the time the exam comes round.
We use may/might (not 'can') + be + -ing to say it is possible that something is happening now or to talk about a possible future arrangement:
• Malcolm isn't in his office. He may/might be working at home today.
• When I go to Vienna I may/might be staying with Richard, but I'm not sure yet.
Could can be used in the same patterns instead of may or might, particularly when we want to show that we are unsure about the possibility.
Notice that we can combine these two patterns to talk about possible situations or activities that went on over a period of time until now:
• David didn't know where the ball was, but he thought his sister might have been playing with it. (= from a past time until now)
We use may/might/could + well/conceivably/possibly + have + past participle  to say it is likely that something would have happened in the past if circumstances had been different, or to say that by some time in the future it is likely that something will have happened.
(Notice that we don't use 'can well (etc.) + have + past participle'):
• I may/might/could conceivably have been tempted to take the job if it had been nearer home, (passive form)
• By this time next week, I may/might/could well have left for Washington.