1. ABILITY
To express ability we use 3 different modal
auxiliary verbs:
• in the present CAN:
– A surgeon can transplant a human face.
• in the past COULD:
– In the 18th C. doctors could operate, but
without anaesthetic.
• in the future WILL BE ABLE TO:
– Robots will be able to operate by the 22nd C.

2. POSSIBILITY
• MAY when it´s possible that something happens
in the future (there are a lot of possibilities):
– Sam may pass his driving test because he did
well during the driving lessons.
• MIGHT when something is possible but it´s
difficult to happen (few possibilities):
– Valencia CF might win the league, but it would
be miracle.
– John could join our team next season, but we
don´t know yet.

3. CERTAINTY
• MUST when we are sure that something
is true or it is happening (surely, definitely):
− You must be joking because I can´t believe
what you´re saying.
• CAN´T when we are sure that something is not
true (impossible, definitely not):
– This song can´t be yours. It´s excellent.

4. ADVICE
• SHOULD/SHOULDN´T:
– You should go to the doctor if you feel ill.
– Mike shouldn´t be so impolite to his mother.
OBLIGATION
• MUST: the obligation comes from the speaker:
– I must do the shopping today.
• HAVE TO: the obligation comes from others (strong obligation):
– You have to be on time to lessons.

5. NO OBLIGATION
• DON´T/DOESN´T HAVE TO: it isn´t necessary:
– You don´t have to wear a skirt (if you don´t
want to).
– Mike doesn´t have to get up at seven todat. It
´s Sunday.
PROHIBITION
• MUSTN´T: it isn´t permitted:
– We mustn´t smoke in hospitals or schools.