Ex6:
1/ We won't go out unless it stops raining
--> We won't go out if it doesn't stop raining
2/ I will call the police unless you give back my bicycle
--> I will call the police if you don't give back my bicycle

Ex 4:
4. It's a long time since I last watered the tree
5. It's a week since it last rained in Ha Tinh
6. Mr. Hoa has worked in this company since 2006
7. When did you learn English?
8. How long is it since the first time you used this computer?
9. Minh has collected stamps since 2000
10. I haven't written to my pen pal since July
11. Ha used to go to school late
12. My father didn't use to smoke