could

Could is an auxiliary verb, a modal auxiliary verb. We use could to:

Structure of could

subject + auxiliary verb
could
+ main verb

The main verb is always the bare infinitive (infinitive without to).

  subject auxiliary verb
could
main verb
+ My grandmother could swim.
- She could not walk.
couldn't
? Could your grandmother swim?

Notice that:

The main verb is always the bare infinitive (infinitive without to). We cannot say: I could to play tennis.

Use of could

could for past possibility or ability

We use could to talk about what was possible in the past, what we were able or free to do:

We use I (positive) and couldn't (negative) for general ability in the past. But when we talk about one special occasion in the past, we use be able to (positive) and couldn't (negative). Look at these examples:

past
general specific occasion
+ My grandmother could speak Spanish. A man fell into the river yesterday. The police were able to save him.
- My grandmother couldn't speak Spanish. A man fell into the river yesterday. The police couldn't save him.

could for requests

We often use could in a question to ask somebody to do something. The use of could in this way is fairly polite (formal):