根据句意和中文注释，写出所给单词的适当形式。

1. It is very important for children to learn to be ________(诚实的).

2. Don't leave the child ________(by oneself) near the river. It's dangerous.

3. It is a________(not useful)book.so I throw it away.

4. I couldn't ________(相信) my eyes when he came to my home.

5. The Greens will have a ________(令人愉快的) trip because they plan everything carefully.

一根木头，要把它平均分成6段，每锯下一段要8分钟，锯完一共要花48分钟．______．