Introduction:

Modals are used to modify the tonality of sentence.
Could indicates possibility. Should indicates obligation. Would indicates condition (often used with 'if).

Structure:

Subject + modal + verb / Modal+subject+verb? / Negatives with not or n’t would, would not, wouldn’t / could, could not, couldn’t / should, should not, shouldn’t

Examples:

I would (would not, wouldn’t) like to go.
Would you like to go?
I could see
Couldn’t I see?
I should not go.
Shouldn't I go?