Chapter 3 (Prohibition and Suggestion)

if it is strongly prohibited we use "must not" be + not allowed to +V1

be + not allowed to +V1 

be + not permitted to + V1 

be + prohibited to + V1

should

would better

Situation: your friend has many tasks to do (you should start to do your task immediately) 

Situation: your mom is almost late to work (you'd better leave now, mom, or you'll be late

Situation: your sister feel bored (we'd better go to the park or just do window shopping at the mall)

Situation: your mother got stomachache (I think we should go to the doctor now)

ETC