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Modal verbs:

Might, may, could, must and can’t

Meaning and use

We can use the modal verbs might, may, could, must and can’t for talking about what we think is possible or true in the present. We don’t know for sure, so we make guesses and suggestions using the information that we have. Usually you can use might, may and could in the same way.

  • There might/may/could be life forms on Mars.
  • There’s some gas on the planet that may/might/could suggest this.

We can also use might, may and could for talking about what we think was possible in the past.

  • NASA says it may/might/could have been suitable for life in the past.

Might not (mightn’t) and may not are used for talking about negative possibility.

  • Their information might not/may not be correct.
  • They mightn’t have got correct data.

But couldn’t is different. It means that something is completely impossible.

  • Other scientists say that there couldn’t be life on Mars.
  • The gas couldn’t be coming from living organisms.

We use the modal verbs must and can’t in the present and the past when we believe strongly that something is certain.

  • Oh, it can’t be true! (I believe strongly that it isn’t true).
  • There must be another explanation. (I believe there’s another explanation.)
  • They must have made a mistake! (I believe they have made a mistake.)

Form

Present positive

The present positive is might / may / could / must + the infinitive of the verb be or another verb. Or the continuous be + verb-ing.

  • The information might be correct.
  • They must know that it isn’t possible.
  • Bacteria-like organisms could be producing the gas.

Present negative

The present negative is might not / mightn’t / may not / can’t / couldn’t + the infinitive of the verb be or another verb. Or the continuous be + verb-ing.

  • Their information may not be correct.
  • They can’t have got the correct data.
  • The gas couldn’t be coming from living organisms.

Past positive

The past positive is might / may / could / must + have + been or the past participle of another verb. Or the continuous been + verb-ing.

  • There must have been water on Mars in the past.
  • Something might have existed there.
  • Who knows what could have been living in that lake?

Past negative

The past negative is might not / mightn’t / may not / can’t / couldn’t + have been or the past participle of another verb. Or the continuous been + verb-ing.

  • There may not have been anything in the lake.
  • Scientists can’t have found anything definite.
  • They mightn’t have been looking in the right area.

Take note: questions

We don’t usually form questions about what is possible or true with might, may or must. It’s more common to use could or do you think …?

  • Could the data be incorrect? Do you think it’s incorrect?

Take note: can’t and couldn’t

We can use can’t have and couldn’t have in the same way.

  • They can’t /couldn’t have found anything definite.

Take note: mustn’t

We don’t use mustn’t to make guesses about what is possible or true. We use can’t instead.

  • These figures don’t add up. They can’t be correct.

NOT: They mustn’t be correct.