Can and could are modal (auxiliary) verbs. They can express ability. Notice that modals have only ONE FORM.
They are not conjugated, so the verb following a modal is in the base form:
She can type very fast. (She has the ability to type fast)
Sam could play the piano when he was just 5 years old! (He had the ability to play the piano when he was 5 years old)

Use can to talk about ability in the present. The negative form of can is can’t:
He can sing, but he can’t dance.
My daughter can tie her own shoelaces.
A: How many languages can you speak? B: I can speak English and French, but I cannot speak Spanish.

Use could to talk about ability in the past. The negative form of could is couldn’t:
I could read and write before I started school.
Sam is sorry he couldn’t come yesterday. He had a last-minute meeting.

Use the appropriate form of to be able to to express ability in all other tenses.
She moved to Shanghai two years ago, but she hasn’t been able to learn the language yet. She feels so frustrated.
You will be able to pass the test if you study hard.
Luckily, all the passengers were able to escape when the bus caught fire.