"Must" is like something is really required, 100% needed to have done. MUST = HAVE TO, but not always:
I must = I think inside I need to do this (inside order)
I have to = it is told me to do (outside order)
Should - "You should wash your nose, it's dirty" is like an advice to do something.
Should = ought to
Present: MUST, but past - had to
Presend: HAVE TO, and past - also had to.
Can is present tense, could - past.
Difference between could and would:
The rate is amount of politeness:
Can < Could < May < Would
Could is in questions really polite, but it is also (see above) past form of "can".
Would is the most polite way of asking, supposing and everything.
It would be a pleasure.
Would it have been Alex?
I was wondering wheter you would open the windows.
Excuse my typos, I answer on phone.