Modal verbs

We can use can,could or may to make a request.

Can I vahe som more cake? (informal)

May I have a glass of water, please? (more polite)

Could I use your pen,sir? (more formal)

- We use can and be able to to express general ability in the present. He can/is able to cook well.

- We use could to express general ability in the past . He could run very fast when he was younger.

- We use could to talk about specific ability in the past with following verbs:hear,see,feel,taste, smell,remember,understand,decide etc.

Note: we use couldn't-wasn't/weren't able to express specific negative ability in the past.

I couldn't/wasn't be able to fifnish my task on time.

He could hear the birds singing.

- We use could to express specific possibility. He could come to the party.(It's a specific possibility)

-We use can to express general possibility. It can be hot in summer. (It's a general possibility)

We use don't have to express lack of necessity.

You don't have to come to the meeting. (It's not necessary)