Rachel doesn't want Emma to go to the playground because...
a- Rachel thinks the baby will fall from the swing.
b- Emma is sick.
c- Rachel had a traumatic incident with a swing.
d- Ross usually forgets Emma everywhere.
Modal verbs are special verbs which express different kinds of modality: obligation, prohibition, possibility, advice or permission.
They use the particle 'not' to form the negative form:
I can't go to the party.
She shouldn't arrive late.
They are placed at the beginning in questions:
Can I go to the toilet?
Could you see the films?
They don't usually have other tenses. Only can (could, in the past, but be able to for other tenses) and have to (which has all the tenses). But should or must don't have other tenses.
Modal verbs are always followed by an infinitive without to, except have to:
She can run fast.
They should be careful with the pet.
We have to pay for the trip.
Modal verbs: can, could, have to, must, should