Who said English is not fun?

Most children when asked what they would like to be when they grow up, they say they would like to be a doctor. But as any other profession, being a doctor is not very easy. It's a profession that requires a lot of resposibilities and dedication. Especially because doctors deal with lives.
Using the modal verbs ( HAVE TO / DON'T HAVE TO / MUST / MUSTN'T) write about some of doctor's obligations and prohibitions.
Eg: They have to study hard.
They must do wherever they can to save people's live.