must的过去式和过去分词怎么写

　　You must signal which way you are going to turn.

　　你要朝哪个方向转，必须发出信号。

　　You're a great help, I must say!

　　我得说，你可没少帮忙！

　　There must be a way around the problem.

　　肯定有解决这个问题的办法。

　　The work must be completed within a certain time limit.

　　这项工作必须在一定期限内完成。

　　There is a growing realization that changes must be made.

　　越来越多的人认识到改革势在必行。