You can express « necessity » using the verb have to, plus the infinitive.

HAVE TO
Subject + have/has to + infinitive = necessity

EXAMPLE
I have to greet clients.
She has to arrive early to work.
They have to finish the project tonight.

NEGATIVE
Subject + do/does/did + have +not (‘nt) + infinitive

EXAMPLE:
I don’t have to work at the office every day.
We don’t have to go to work on weekends.

CAREFUL!
“Have to” can be replaced with “need”, which also expresses necessity.

EXAMPLE
She needs to finish the project.
They don’t need to work today.