shall的过去式是should，但是怎么和情态动词的should区分啊，答得好我会提高悬赏！！！
shall
将来时,用于第一人称：I shall be back in a minute.
用来表示征求意见：Shall I go with you?
用在第二、三人称,表示命令,警告,允诺等.He shall answer for it!
should应该
You should be responsible for yourself.
表猜测It should be true.大概有70-80%的把握.
should have done本应做某事,语气中有责备.
You should have come here earlier.你本来应该早点到的.