must后面加什么动词形式

　　You've been a great help, I must say.

　　依我看，你真是帮了个大忙啊。

　　I must go to the bank and get some money.

　　我得上银行取点儿钱。

　　He must be 70 if he's a day!

　　他至少70岁了！

　　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！