Step on the grass.
You mustn't step on the grass.
Wear a uniform in Hogwarts.
You must wear a uniform in Hogwarts.
Touch the paintings in the museum.
You mustn't touch the paintings in the museum.
Wear shinpads in football.
You must wear shinpads in football.
Wear a coat if it is cold outside.
You should wear a coat if it is cold outside.
We use should / shouldn't...
Advice / Opinion. Recomendation.
Keep your bedroom tidy.
You should keep your bedroom tidy.
Take pictures.
You mustn't take pictures.
Watch TV from very close.
You shouldn't watch TV from very close.
Eat fruit and vegetables.
You should eat fruit and vegetables.
Wash your hands before eating.
You should wash your hands before eating.
Shout at your friends.
You shouldn't shout at your friends.
Touch the oven when it is hot.
You shouldn't touch the oven when it is hot.
We use must / mustn't...
Obligation / prohibition. Rules, laws (no choice).
Play with your food.
You shouldn't play with your food.
Shower before entering the swimming pool.
You must shower before entering the swimming pool.