Necessary & Suggestion

1. Necessary (must, have to, need to)
      You must be at least 18 to drink.

2. Not Necessary(don't must, don't have to, don't need to)
       You don't have to be 20 to drink.

3. Suggestion(should, ought to, had better)
       You should ware some jackets.
       You had better ware sunglasses in the US. (strong suggestion)

4. Not Suggestion(Shouldn't, had better not)
       You shouldn't take too much money.

5. Not Allow(mustn't)
       You mustn't eat food in MRT station.