What's difference and usage: Could and must

Hi Everyone,

What’s main difference between these words: ““could “” and “” must””.which appropriate place put these modal verbs.

kindly support for these issues

Thanks

Bijo

Hi,

‘Could’ has the sense of either ‘would be able to’ or ‘was able to’ depending on the context:

When I was a child I could (was able to) swim like a fish.

Today I honestly don’t think I could (would be able to) swim even one length of the pool.

‘Must’ is a verb suggesting necessity or compulsion and can often be replaced by ‘have to’

I must (it is necessary for me to/I have to) leave now in order to catch my train.

Alan