REWRITE: You are obliged to wear a helmet when you ride a motorbike.
You must/have to wear a helmet when you ride a motorbike.
FILL IN THE GAP: Don't forget the umbrella! I .................. rain today.
It might/may/can rain today.
FILL IN THE GAP: I .................. swim when I was 10 years old.
I could/was be able to swim when I was 10 years old.
FILL IN THE GAP: ....... I come in, please?
Can/Could/May I come in, please?
REWRITE: It is possible I pass the English test. I....
I might/may/can pass the English test.
REWRITE: When I was younger, I had the ability to ride a monocyle.
When I was younger, I could/was able to ride a monocycle.
REWRITE: It is prohibited to smoke here. You....
You musn't smoke here