Uses
This modal verb is used to talk about ability in the present.
Can
Turn this sentence into negative:
I can do backflips
I can't do backflips
I am can run three miles a day
I can run three miles a day
This modal verb is used to give advice
Should
Turn this sentence into interrogative:
It shouldn't be this way
Should it be this way?
He shoulds go to the doctor
He should go to the doctor
This modal verb is used for possibilities
Could
Turn this sentence into interrogative and affirmative:
You mustn't feed animals
You must feed animals
Must you feed animals?
They must to like eating out in Mondays
They must like eating out on Mondays
This modal verb is used when something is mandatory
Must
Turn this sentence into a polite request:
I can go to the bathroom.
Could I go to the bathroom?
Could you help me painting that fence?
Yes, I could!
Could you help me painting that fence?
Yes, I can. / Sure / Any other answer that doesn't use "could"
This modal verb should not be used when answering
Could
Turn this sentence into one that uses each modal verb in each form.
He bakes cakes every morning.
He can bake cakes.../ He can't bake cakes.../ Can he bake cakes?
He could bake cakes.../ He couldn't bake cakes.../ Could he bake cakes?
He should bake cakes.../ He shouldn't bake cakes.../ Should he bake cakes?
He must bake cakes.../ He mustn't bake cakes.../ Must he bake cakes?
I could hold the door for that person, they are carrying to much
I should hold the door for that person, they are carrying too much.