根据句子意思，用所给词的适当形式填空。
Unluckily, most of the students fell off the rocks and hurt ________________(they).
I don’t think his advice is ________________(use), but he thinks it’s the most important of all.
The Internet makes               ( communicate) much easier in the modern world.
The Greens ________________ (see) the film twice, but they want to see it again.
When he walked past the classroom, he saw a group of boys ________________ (study) the Chinese history.