Could, mustn't, needn't

Must - mustn't

Must + infinitive = it is necessary to do something. For past tenses use had to.

positive - must I must stop the car. You must stop the car He / She / It must stop the car. We must stop the car. You must stop the car. They must stop the car.

negative - mustn't I mustn't stop the car. You mustn't stop the car He / She / It mustn't stop the car. We mustn't stop the car. You mustn't stop the car. They mustn't stop the car

Mustn't + infinitive = it is necessary not to do something.

Need - needn't

Need

Need + infinitive = Require something because it is very important.

Needn't

Needn't + infinitive = Require something because it is not very important.

I needn't stop the taxi (because it is not very important to do so).
You needn't stop the taxi.
He / She / It needn't stop the taxi.
We needn't stop the taxi.
You needn't stop the taxi.
They needn't stop the taxi.