must，could，don't，can't，let's
（1）-Is that man over there your uncle ？
-It

can't

can't

be him．He is in Beijing now．
（2）-Whose bike is this？
-It

must

must

be Mary's．In our class，only she rides to school．
（3）-

Let's

Let's

go for a picnic tomorrow．
-That's a good idea．
（4）

Don't

Don't

talk loudly in public places，Jill．It's not polite．
（5）There

could

could

be rain today so you'd better take an umbrella when you go out．