Could expresses possibility, while would expresses certainty and intent. A good way to remember the differences between these two words is simply to bring each word back to its root verb. Could is the past tense of can. Would is the past tense of will.

Could Vs to be able to?

We sometimes use be able to instead of "can" or "could" for ability. Be able to is possible in all tenses - but "can" is possible only in the present and "could" is possible only in the past for ability. ... So we use be able to when we want to use other tenses or the infinitive.

When should I use could or would?

Could, would, and should are all used to talk about possible events or situations, but each one tells us something different. Could is used to say that an action or event is possible. Would is used to talk about a possible or imagined situation, and is often used when that possible situation is not going to happen.