根据句意，用括号中所给单词的适当形式或句子的语法要求填空。
1. Playing too many computer games is    ▲   (harm) to you.  
2. We should speak to the old man    ▲   (polite).    
3. —How many    ▲   (tooth) does a dog have?
—Sorry, I have no idea. Let’s search on the Internet.
4. A computer is    ▲   (expensive) than a TV set. 
5. Have you read    ▲   (today) newspaper? 
6. I    ▲   (not go) to bed until my mother came back at ten last night.
7. You need    ▲   (return) the comic book to the library at once.
8. My sister    ▲   (be) a singer since she left the school.