这里分开说
must 和may
must 表示主观上的义务和必要,主要用于肯定和疑问句,译为“必须……,得……,要……”
must表示推测时,用在肯定句中表示较有把握的推测,意为"一定”
may 在表示推测时,表示没有把握的推测
should和would
should 以前只用于第一人称,随着英语的同化,现在should 或 would 这两个词中的任何一个都可以用于第一人称,表示条件式中的将来,如：
If I had known that,I would (or should ) have answered differently.
但在第二人称或第三人称中只用would
如：
If he had known that,he would (not should ) have answered differently