1. Use can to talk about general possibilities, which are true for many people or at many different times:

• He can be really mean sometimes.
• Summers can be really hot.
• It can take two hours to get there by train.

2. Use could, might or may to talk about specific possibilities in the present/future. Specific = true at one moment, in one situation:

• She might be there tomorrow.
• He may not be able to help you.
• It could take you two hours to get there by train.

3. Use could have, might have or may have to talk about possibilities in the past if you aren’t sure what happened:

• He might have forgotten about it.
• It could have got lost in the post.
• They may have left already.

4. Use could have to talk about a possibility in the past if you know what happened, but you want to express something which could have happened if things had been different:

• You should be more careful! You could have really hurt yourself.
• I could have handled that situation much better.
• They could have done more to help us.

In sentences like:

• This summer might be really hot.
• It could take us a few hours to finish this.
• They might have missed the train.

We use the modal verbs to express possibility: something which could happen in the future, can be true generally, or might have happened in the past. You’ll learn more details about this during this class.

1. Talking about a general possibility in the present or future

To talk about something which is generally possible, you can only use one verb: can

What does ‘generally possible’ mean? It means you’re talking about something which is true for different people at different times. You aren’t talking about one action, one moment or one event.

For example:

• It can take two hours to get there by train. –> This is a general possibility—it can take two hours whenever you go.

• Summers can be really hot. –> Every year, not just one year.

Learn more phrases with this lesson from Oxford Online English: Talking About the Weather.

2. Talking about a specific possibility in the present or future

What about specific possibilities? You can use three verbs: could, may, might. For example:

• It could take you two hours to get there by train. –> This is specific: it could take you two hours, not someone else.

• This summer might be really hot. –> Again, this is specific: we’re talking about this summer, not every summer.

It doesn’t matter which verb you use. You can say:

• This summer might be really hot.
• This summer may be really hot.
• This summer could be really hot.

All of these are possible, and the meaning is the same. Remember that you can’t use can for a specific possibility. Also, while you can use all three of these verbs, may is more formal, and less common in spoken English. Might and could are more common.

3. Talking about the past when you don’t know what happened

Talking about possibilities in the past is complicated, because there are two different situations. Look at these examples:

• I don’t know where they are—they could have missed the train.
• You were lucky—if you’d been one minute later you could have missed the train.

In both sentences we use the verb could to express that something was possible, but they aren’t the same. Can you tell the difference?

In the first sentence:

• I don’t know where they are—they could have missed the train.

The full meaning is: I don’t know what happened, but it’s possible that they missed the train, although there are also other possibilities.

In this case, you can use could, may or might + have + past participle to talk about possibilities in the past. For example:

• He might have forgotten about it. –> I don’t know if he’s forgotten or not, but it’s possible.

• It could have got lost in the post. –> I don’t know what happened to it, but this is one possibility.

• They may have left already. –> I don’t know if they’ve left or not, but it’s possible.

4. Talking about the past when you know what happened

Now, let’s look at our second sentence:

• You were lucky—if you’d been one minute later you could have missed the train

The full meaning is: I know what happened, and I know you didn’t miss the train, but if things had been slightly different, it’s possible that you would have missed the train.

The important point here is that in the first sentence, we don’t know what happened, but in the second sentence, we know what happened.

In this situation, you can only use could have + past participle. You can’t use may or might. For example:

• You should be more careful! You could have really hurt yourself. –> You didn’t hurt yourself, but it was possible.

• I could have handled that situation much better. –> I didn’t handle it well, but if I could do it again, I could do it better.

• They could have done more to help us. –> They didn’t help us very much, even though it was possible for them to do so.

5. Negatives

You’ve already seen that in positive sentences, may, might and could generally have the same meaning. However, in negative sentences, could is different from may and might. In fact, in negative sentences, could has the same meaning as can.

Look at two sentences. Can you see the difference?

• He might/may not know about it yet.
• He couldn’t/can’t know about it yet.

The first sentence, with might/may, means It’s possible that he doesn’t know. Maybe he knows, and maybe he doesn’t.

The second sentence, with can/could, means It’s impossible that he knows. In this case we’re certain that he doesn’t know.

Another small point: cannot and could not are usually shortened to can’t and couldn’t. May not and might not are always used as two words—we don’t use the short forms.