有關情態動詞點用、點解?

1in fact, you can just memorize the meaning of them and you can use them in most ways.
eg can(現在式,解可以)=could(過去式)= is/am/were able to(過去式/現在式/其它時態都好似得)
shall,will(將來式，解將會)= would (過去的將來式)
may((現在式,解可能)=might,(過去式)
should,(現在式,解應該)ought(現在式解義務上應該做的)
must= have/had to(過去式/現在式/其它時態都好似得)

2 extra function
would can express order/question in a polite way.
i would like a big one please.
would you like having tea?

3of course, there are a lot of other usages. you have know it when contacting more english but can not say all of them here.

你所講述的全是aux. 助動詞.是與v.一起運用,去[幫助/表現]v.的動作.

例: Will you go?你會去嗎? .......You will go. 你會去!