何时用"shall"何时用"should"?

1.should
(1)should可用来表示责任.义务或提出劝告,可用与各种人称,如:
You should follow our advice.
(2)谈将来可能性很大的事,但不能用与过去.如:
Our guests should be home by now.
(3)should和 why, how连用表示不能理解的意思,有时带有气恼口气,如:
How should I know that he's a jerk?
(4)用与第一人称可表示委婉地陈述看法.如:
I should be glad to have a try.
2.shall
(1)在疑问句中适用于第一.三人称表示征求意见.询问或请示.如:
Where shall I wait for you?
(2)用于第二.三人称,表示决心.命令.警告.允诺等.如:
You shall do as you see me do.

另外,should是shall的过去式

should 表示委婉语气
should 是 shall的过去式

需征求某人意见时多用shall,表达委婉的语气。类似的还有would you...?