1. 用"am"、"is"、"are"、"was"、"were"填空。

1. （1） The rivers dirty before, but they clean now.
3. （2） —Let's put the trash there.
   —Oh, it heavy. I can't take it. I not strong.
5. （3） Listen! The birds singing. The cow dancing.
7. （4） The weather hot yesterday, but it cool now.
9. （5） — your father busy yesterday evening?

   —No. But I busy yesterday evening.