These three modal verbs are related, but they do different things:

Cambridge lists could, should, and would among the core modal verbs used to express meanings like possibility, advice, and hypothetical situations, and British Council explains that could and would are often used for polite requests and offers.

✅ I would go if I had time.

Here, would shows a hypothetical situation. British Council notes that would is commonly used to talk about imagined situations and for politeness.

✅ When I was five, I could swim.
✅ We could leave now, if you want.

In the first sentence, could expresses past ability. In the second, it expresses possibility. British Council and Merriam-Webster both support those uses.

✅ You should get some rest.

Here, should expresses advice or recommendation. Cambridge’s grammar guidance includes should among the modal verbs used for this kind of meaning.

✅ I would help, but I couldn’t stay long, and you should call first.

This sentence shows the three jobs clearly:

The most common mistake is treating these three words like interchangeable versions of maybe or polite English.

They are not interchangeable.

For example:

❌ You would study more.
✅ You should study more.

If you are giving advice, should is the better choice.

Another common mistake:

❌ When I was a child, I would swim at age four.
✅ When I was a child, I could swim at age four.

If you are talking about ability, could is usually the right word. British Council explains that could is used for ability, while would is more connected to hypotheticals and politeness.

Use this quick rule:

A simple memory trick:

Or even shorter:

would imagines
could allows
should advises

That will get you the right choice most of the time.