must的過去式和過去分詞怎么寫

　　You must signal which way you are going to turn.

　　你要朝哪個方向轉，必須發出信號。

　　You're a great help, I must say!

　　我得說，你可沒少幫忙！

　　There must be a way around the problem.

　　肯定有解決這個問題的辦法。

　　The work must be completed within a certain time limit.

　　這項工作必須在一定期限內完成。

　　There is a growing realization that changes must be made.

　　越來越多的人認識到改革勢在必行。