Here are the correct answers to fill in the blanks with "since" or "for":

1. Mark has used the laptop **for** three hours.
2. How many times have you read the book **since** you bought it?
3. Mary hasn't eaten fish **since** Thursday.
4. We've known each other **for** ages.
5. My sister has owned the fashion shop **for** 3 years.
6. It's three years **since** Trang started learning English.
7. Peter and Linda have been married **for** a year.
8. She has been to Paris **for** a week.
9. He has stayed here **for** three days.
10. I have worked as a teacher **since** 2018.