Modal verbs are used to express ideas like ability, permission, obligation, and possibility. The main modal verbs discussed are: - Can expresses ability or permission. It is also used for possibility or impossibility. - Could expresses ability in the past or possibility. It also makes polite requests. - Have to expresses obligation in the affirmative and interrogative but lack of obligation in the negative. - May expresses possibility or permission, especially in formal contexts. - Might expresses small possibility. - Must expresses present or future obligation. Mustn't expresses prohibition. - Should gives advice.

1. MODALS VERBS
Modal verbs are auxiliary verbs that are used to express modality, i.e., ability,
likelihood, permission, prohibition, obligation, necessity, etc.
Exception: Although have to is used for expressing obligation or lack of it (in negative),
it is not a real modal verb; it needs therefore the auxiliary verb do/does for making the
sentence interrogative or negative. E.g.: You don’t have to go to the gym for losing
weight.
Modal verbs are followed by a bare infinitive, so you don’t have to write the –s of the
third person: She can do mathematical operations very fast.
CAN
Ability
I can hold my breath for three minutes, but I can’t swim a long time.
Can you ride on bike?
Permission (asking, giving or denying permission)
You can have a break now.
You can go to the cinema with your friends.
Can I open the window?
Possibility // Impossibility
Mark can visit his doctor tomorrow in the morning, but now Dr. Eriksen is busy.
Peter can’t play football this evening. He has to study for his exam.
COULD
Ability (in the past)
When I was fourteen, I could run 20 twenty meters in 5 seconds. (Cuando tenía 14 años,
podía correr 20 metros en 5 segundos)
Possibility
How much costs a flight to New York? It could cost around 500 euros.
Have you got any cash for me? There could be some in my pocket.

2. Making polite requests
Could you mind helping me with this activity?
Could you join us on Saturday?
HAVE TO
In the affirmative and the interrogative form: Obligation
You have to finish your homeworks if you want to go out.
When I was at school, I had to study everything by heart.
You will have to wait in line as everyone is doing.
Did you have to pay any fees for signing up for the gym?
However, if you used have to in the negative, it has the opposite meaning: Lack of
obligation or no necessity
He doesn’t have to hand that document in if he doesn’t want to.
At the 60s, people didn’t have to go to school if they didn’t want to.
MAY
Possibility (in the future)
This treatment may have some side-effects.
Permission (formal)
- May I leave now?
- You may leave when the ring bells.
MIGHT
Small possibility
- You have to buy your ticket. There might not be many left.
- It might be a good concert. I like the group that will play. (Quizás sea un buen
concierto. Me gusta el grupo que tocará).
MUST (for the present and the future tense)
Must
Obligation
Students must write the work by hand, not with the computer.

3. All the passangers must remain seated during the landing.
Mustn’t
Prohibition
Visitors mustn’t touch any painting or work from the museum.
Students mustn’t talk to anybody during the exam.
SHOULD
Advice
- You really should see a doctor about his back problem.