could/ can; would/ will; should/ shall

Could, would and should are the conditional forms of those verbs. A condition must be met for the idea to be operative, or the idea is not operative because the condition cannot be met.

I can swim at beaches without a heavy surf.
I could swim at this beach, if the surf were not so heavy.

We will go to dinner at eigth.
We would go to dinner at eight, if she arrives in time.

You shall finish the project before you do anything else.
You should finish the project, if you expect to do anything else.

Re: could/ can; would/ will; should/ shall


Because the modal verbs have so many areas of meaning, it simply isn't possible or practical to try and describe how they work in one posting, Oriental.

You should ask some questions using some examples. That might be a good start.

For the modal pairings you've given above, the best advice I can offer is that often the first two pairings have the same meanings but we tend to use 'could' and 'would' in situations of greater doubt.

Those same type of connections do not hold in modern English for 'should/shall'.

Ask away. There are tons of ENLs around here ready to offer great advice.