根据句意，用所给单词的适当形式填空。

1. Look, there is a bird _____ (sing) in the tree.

2. The bad air makes my chest hurt and it's difficult for me _____ (breath).

3. They are doing their homework carefully. You'd better not _____ (disturb) them.

4. Litter _____ (influence) the look of our city.

5. It is known that smoking does great _____ (harm) to us.