Read about using can and could for ability. Then, when you are ready, try these practice exercises with can / could.
General abilities (past and present)
Some abilities are general: you have the skill to do it at any time you like. We use can for present abilities and could for past abilities.
I can play the accordion.
I could play the violin when I was a child.
Specific Abilities in the Past
Not all abilities are general. Some abilities happen only once (or a certain number of times) in a particular situation. In positive sentences, we don't say could, we use was able to.
I was very tired but I could stay awake all night. X
I was very tired but I was able to stay awake all night. ✓
If the sentence is negative, we use could or was able to.
I was very tired, so I couldn't stay awake. ✓
I was very tired, so I wasn't able to stay awake. ✓
How to make sentences with can and could
Positive: She can speak 3 languages.
Negative: He can't speak Arabic.
Question: Can you speak any languages?
Positive: They could speak some German when they were 15.
Negative: He couldn't speak any Spanish when he was 15.
Question: Could you speak French when you were 15?
Remember: modal verbs don't need an 's' for he/she/it, and always have an infinitive verb (without 'to') after them.
He cans play the piano. X
He can play the piano. ✓
They can to make delicious cakes. X
They can make delicious cakes. ✓
Note: if you want to talk about abilities in the future or use other tenses, you need to use be able to in the correct tense.
I will be able to speak perfect French one day. (future)
I have been able to swim since I was five. (present perfect)
Now try these practice exercises with can / could.