Here’s a suggested way to fill in the blanks:

1. **for**
2. **since**
3. **when**
4. **for**
5. **for**
6. **since**

So the complete text would read:

"I’ve known David **for** six months. We’ve been good friends **since** last summer. We haven’t had many arguments **when** we met, but are **for** a while now. His parents have lived here **for** a few years. They think David should know more about Vietnamese culture. He hasn’t spoken to me **since** we felt I don’t know what I can do."