Can and Could for Ability
Some abilities are general: you have the skill to do it at any time you like.
I can play the accordion.
I could play the violin when I was a child.
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. ✓
|Present||+||I can speak 3 languages.|
|-||I can't speak Arabic.|
|?||Can you speak any languages?|
|Past||+||I could speak some German when I was 15.|
|-||I couldn't speak any Spanish when I was 15.|
|?||Could you speak French when you were 15?|
Note: if you want to talk about abilities in the future, or use other tenses, you need to use be able to.
I will be able to speak perfect French one day.
I have been able to swim since 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 only 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 couldn't hear him because of the music. ✓
I wasn't able to hear him because of the music. ✓
Practice 'can' and 'able to' Next: modals of obligation