1，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……”

may 在表示推测时，表示没有把握的推测

2，should和would:

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：