Modal verbs like must, have to, and should are followed by an infinitive without "to". Must and have to express obligation, while should expresses suggestions and advice. Negatives are formed with "not" and interrogatives use subject-verb inversion. Mustn't expresses prohibition. Common uses are expressing what is required or advising what may be a good idea.

1. FORM
USES
HAVE TO - MUST-
SHOULD
Esther Martínez

2. - Modal verbs are followed by an bare INFINITIVE.
You must go to the doctor.
- We use “not” to make them negative.
She shouldn´t leave now.
- We use subject-verb inversion in the interrogative.
Do I have to go to school today?
LET´S RECAP: FORM

3. We use “HAVE TO” and
“MUST” to talk about
obligation.
We have to go.
We don´t have to go.
Do we have to go?
HAVE TO and MUST

4. We use “MUSTN´T” to express prohibition.
You mustn´t use your phone at school.
MUSTN´T

5. We use “SHOULD” to give suggestions and advice.
You should go to the doctor.
SHOULD