I think you follow school rules and regulations.

giving advice

You wear your uniform on Saturday.

expressing a lack of obligation

I translate documents from Chinese into Spanish a year ago.

expressing a lack of past ability

I help Jane with her homework.

expressing personal obligation

She be in detention after school for breaking the rules.

expressing external obligation

I reply to my penfriend's letter now because I'm too busy.

expressing a lack of ability

You to talk to your students about the importance of being punctual.

expressing necessity