shall的過去式是should，但是怎么和情態動詞的should區分啊，答得好我會提高懸賞！！！
shall
將來時,用於第一人稱：I shall be back in a minute.
用來表示征求意見：Shall I go with you?
用在第二、三人稱,表示命令,警告,允諾等.He shall answer for it!
should應該
You should be responsible for yourself.
表猜測It should be true.大概有70-80%的把握.
should have done本應做某事,語氣中有責備.
You should have come here earlier.你本來應該早點到的.