Modal verbs

We use "can" to express an ability and "can't" to describe the lack of ability.

e.g.:​ She can speak Spanish but she can't speak Italian.

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We can also use "is/are able to".

To describe a future ​ability we use "will be able to", not can!

We use "could" to describe the possession of an ability in the past.

e.g: He could play the piano at the age of five.

We can use "could" in questions but to describe the successful use of an ability, we use "was/were able to". ​

​We use "could/might have + past participle to describe a past ability which was not used.

e.g.: She could have paid by credit card.

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You could have told me about the party!

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We use "can" to describe things which are generally possible.

​e.g.: Drinks in restaurants can be very expensive.

We use "may", "might" and "could" to talk about specific possibilities.

e.g.: There may be life on Mars. (=perhaps)​

​We use "might" for a less direct question:

e.g.: Might the losses be due to currency fluctuations?

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We use "can't" or "couldn't" for things which we know are impossible:

e.g.: You can't get blood out of a stone.

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When we think that something is possibly not the case, we use "might not" or "may not".

e.g.: The shops may not/might not be open today.

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We use "could" to talk about general possibility in the past.

e.g.: Teachers could be very strict at my school.

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We also use "might have" for a past opportunity and the motive.

e.g.: She might have done it. ​

​We also use "might have" for a past opportunity.

​e.g.: I might have gone to drama school, but my parents wouldn't let me.

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We express a negative deduction about the past with "can't/couldn't have".

e.g.: She can't have fixed the computer.

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In the future we can use "will/"won't be able to".

e.g.: We'll be able to travel to the moon, but we won't be able to travel to Mars.​

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For predictions which are less certain we use "may", "might", or "could".

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We use "may/might have + past participle" to talk about a possible completed action by a time in the future.​

e.g.: Call me next Tuesday, I might have finished the project by then.​

We can use "can/could or "be able to" to describe possible arrangements in the future.

​e.g.: I will be able to see you after the lesson.

Offers: can, could, may, might;

Suggestions: can, could, might;

Requests: can, could, might.​

Asking for and giving/refusing permission

We use "can/can't" for permission granted or refused by the person. - present/future

e.g.: Can I use your phone?

may - formal

might - to ask for permission in a more tentative way;

e.g.: Might I ask the court for an adjournment at this point?​

We use "could/couldn't" to describe general permission in the past.

​e.g.: In the past, British children could leave school at the age of fourteen.

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To talk about permission on a specific occasion in the past we use "was/were allowed to".

e.g.: I was allowed to leave early yesterday.​