Could-Should | Commonly Confused Words

Could

Expresses possibility or permission in the past, also used for polite suggestions or inquiries

She could hear a pin drop in the silent room.If we hurry, we could catch the earlier train.Could you open the window, please?

This word has no antonyms

/ʃud/

Used to indicate obligation, duty, or correctness, typically when criticizing someone's actions; also used to indicate a conditional situation

You know how sorry we should be.'You should not accept a hundred,' said Poirot.That should get at least a mild chuckle.

This word has no antonyms

Both words are used for hypotheticals, but could implies ability while should implies obligation or recommendation.
Remember that could stems from can, indicating capability, whereas should relates to advice or expectation.
Think of could as asking if something is possible, while should asks if it's advisable to do it.