2. I don't like maths. There are several reasons for that
→There are several reasons why I don't like Maths.
3. My friend has decided to buy a motorbike. His bicycle was stolen last week.
→My friend, whose bicycle was stolen last week, has decided to buy a motorbike.
4. The man is my former teacher. He gave you a lift last night(who)
→The man who gave you a lift last night is my former teacher.
5. You should ride your bike carefully, or you may have an accident.
→If you don't ride your bike carefully, you may have an accident.
6. She can't join the party because she has to study.
→If she didn't have to study, she could join the party.
7. I can speak both English and French perfectly. (Bilingual)
→ I'm a bilingual who can speak both English and French perfectly.
8. They are looking for the girl and her dog.They have lost the way in the forest. (that)
→ They are ?
9. I have a friend. Her mum is a volunteer worker (whose)
→I have a friend whose mum is a volunteer worker.
10. Study harder, or you will fail the exam.
→ If you don't study harder, you will fail the exam.