musn't (prohibition)

You musn't smoke here.
You musn't croos the street now.

Need to

If you need something please call me.
I need a free day.

Need to

Don't have to/ don't need to/ needn't (No obligation)

I don't have to go to the university today because is optional.
You don't have to pass the test.
We needn't come to my house, we can go to your house.
You didn't need to wake up this morning because is sunday.

Must and have to (obligation)

You must study if you want to pass the exam.
I have to go to the work today.

countable and uncountable nouns

I don't have money, but you have much money.
There is some water in that pitcher.
I have a beautiful black dog.

singular and plural nouns

When I was 11 years old I didn't have a lamp in my room, and now I have two lamps.
I don't like to clean the dishes.
Sometimes I buy one pen, sometimes I buy more than one pen.
I like my city, I think is one of the best cities in the world.