Modal Verbs

MUST 
This auxiliary may indicate or can be used for an obligation.

Examples:

• You must see a doctor! you have a cold.
• You must do your homework.  

NEED
We will use the word "Need" to say we need to do something.

Examples:

• I need some water please.  
• I need you to go to the fruit store to buy some oranges.

HAVE TO
We will use the auxiliary "have to" to express an obligation but not as rigid as someone or something must have asked us to do.

Examples:

• I have to go to the park with my friends. 
• She has to clean up her house.