Introduction The present course paper deals with the study of the

II. 2 Demand
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 them¹⁶.
He cans play the piano.
He can play the piano.
They can to make delicious cakes.
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.
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.
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.
Can
We use can when we speak about general ability in the present.
Could
We use could when we speak about general ability in the past.
Tommy can swim.
I can play the guitar. Can you play? Picasso could paint when he was two.
I could swim before I could walk.
The negative of can is can't (cannot):
I can't ski and I can't skate. The negative of could is couldn't (could not):
I couldn't swim until I was ten.
We also use can to speak about specific situations in the present:
I can hear you, but I can't see you.
Can you hear me? ∼ Yes, I can. We often use was able to speak about a specific situation in the past¹⁷:
We fixed the car and then we were able to drive home.
We prefer could to speak about a specific situation in the past when we use these verbs: see, hear, feel, smell, taste, remember, believe, understand, and decide.
They could smell smoke.
I could understand him perfectly.
Questions about ability in the present:
How many languages can you speak?
Can you name all the capitals of Europe? Questions about ability in the past:
Could you write before you started school?
Could you ride a bike when you were small?