We often use could to make suggestions:

A:

Will’s party is fancy-dress.

B:

It’s Halloween.

C:

Oh right. I could go as Julius Caesar.

B:

Again? How many times have you done that?

A:

I’ve got to be in the meeting at 10 and the train doesn’t get in until 10.15.

B:

Could you get an earlier train?

We use could to ask for permission. Could is more formal and polite than can:

Could I ask you a personal question?

When actual past achievements are mentioned, we usually use was/were able to or managed to but not could in affirmative clauses. This is because they are facts, rather than possibilities:

I was able to/managed to buy a wonderful bag to match my shoes.

Not: I could buy a wonderful bag to match my shoes.

We hired a car and we were able to/managed to drive 1,000 miles in one week.

Not: We hired a car and we could drive 1,000 miles in one week.

We use could to talk about past ability:

When I was young, I could easily touch my toes.

We use could have + -ed form to talk about possibility in the past:

I could have been a lawyer.

They could have taken a taxi home instead of walking and getting wet.

Janette couldn’t have done any better.

When we want to guess or predict something, we use couldn’t as the negative form of must. We use couldn’t have + -ed form as the negative form of must have + -ed. Couldn’t and couldn’t have + -ed form express strong possibility:

She must have made a mistake. It couldn’t be true.

A firework couldn’t have done all that damage.

We use could to refer to single events that happened in the past, with verbs of the senses (smell, taste, see, hear, touch, etc.) and mental processes (think, believe, remember, understand etc.):

The food was terrible. I could taste nothing but salt.

We knew they were in there. We could hear voices inside.

He came and spoke to me, but I couldn’t remember his name.

We use could when reporting clauses with can as past events:

They told us we could wait in the hallway. (The original words were probably: ‘You can wait in the hallway.’)

She said we could book the tickets online. (The speaker remembers hearing ‘You can book the tickets online.’)

We often use could have + -ed form to express disapproval or criticism:

You could have called to say you would be late. (You didn’t call – I think you should have called.)

You could have tidied your room.

We use could have + -ed form to talk about things that did not happen and sometimes to expresses regret:

He could have been a doctor.

I could have been famous.

We often use the expression how could you/she/he/they? to show disapproval (to show that we don’t like what someone has done):

Grandfather, how could you? How could you leave me?

How could you have gone without telling me?

A:

We had to give away our dog when we moved to England.

B:

Oh, how could you?