1. Most people think it worthwhile spending some time on the internet.
2. Our teacher has decided to devote his life to education. （当然,教育事业可直译为：educational cause,但个人觉得此处不宜这样翻译）
3. In my opinion, he is an outspoken person. That is (to say), he always means what he says.
4. I didn't know that Jack was also in Paris （我觉得这样翻译比：Jack had also gone to Paris更好）, I just met him by chance (或 we just met by chance).
5. If you run into any difficulties while doing the exercises, you can refer to your notebook.