You see someone littering in the park. What should you do?
I should warn him/her.
Your father didn't turn off the lights when he left the room. What should you do?
You should remind him to turn the lights off./ You should turn them off.
Your sister didn't seperate your garbage for recycling. What should you say to your sister?
"You should sort out your rubbish. / You should seperate your garbage."