Could is a modal verb. It is a helping verb that we usually use with a main verb in a sentence to express ability, possibility, necessity or permission.
Some of the modal verbs are should, would, must, have to, might, etc. Modal verbs give different meanings to the main verbs.
For now, we will only have a look at the use of could for general ability in the past.
1. When we want to talk about ability in the past, we use could. It is the past form of can.
2. We use couldn’t when we didn’t have the ability to do something in the past
We add could after the subject and use the bare infinitive form of the verb to form positive sentences.
Subject + could + bare infinitive
I could play the guitar when I was 15.
We use could not with bare infinitive after the subject to form negative sentences.
The short form of could not is couldn't and we usually use couldn't in negative sentences.
Subject + could not + bare infinitive
He couldn't ride a bike when he was a child.
We use could before the subject to form questions.
Could + subject + bare infinitive ?
Could you play chess when you were 10 ?