The most difficult thing for me in English grammar was the Modality of the verbs. That is why I want to teach you, and prevent you from the similar issues as I had in my childhood.

Remember that we do not conjugate these verbs:

I can, she can, he must, you should, etc.

Exception to this little rule are the verbs “to be”, and “to have”:

I am to eat more protein to be stronger.

You have to stay here.

Modal verbs are just auxiliary verbs. Their difference is that they express likelihood, possibility, ability, obligation, permission, and so on. Depending on their expression we can choose one particular modal verb to say something like in the supermarket! What I mean: when you need to express permission you do not take the verb that shows ability.

The list of modal verbs is not long, but we should pay attention on them: can (could), be able to (was/were able to), may (might), must, have to (had to), be to (was/were to), should, ought to.

1) We use can, could, and be able to for:

Ability, possibility (real or supposed), doubt about the possibility of something, reproach, lost opportunity, demand, permission, prohibition, supposition, intention (only with could).

In the Present it will be “can,” but in the Past or Future we will use “could”, and “be able to”.

• Look, my cat can stand on two paws!
• When he was a child, he could do more exercises.
• You can go to France by train.
• Can it be alive? I doubt if it is possible.
• I could have learnt five more languages by this time. But I did not.
• Can you give me your mobile for a second, please?
• You can’t smoke here.
• You can walk in this street. It is a pedestrian street.
• He painted her portrait so that he could see her every day.
• Everyone was able to escape.

2) We use May, might for:

Real possibility, reproach (only might), supposition with lack of confidence, demand, permission, prohibition, wish. This verb expresses less confidence than the other ones:

• May I come in? I am so late that I am not sure if it is possible. – No, you may not.
• He may come today, but I am not sure.
• You might have thought about the result before!
• Their invention may be useful in some scientific fields.
• May you be happy!

3) We use two verbs to express necessity, duty, responsibility, a piece of advice, strict prohibition, confident supposition, insistent demand: must, and had to for the Past variant.

• You must do it today.
• You mustn’t tell this to anyone!
• You must visit Paris. I insist on this.
• I guess, they must have come to this place without me.
• He had a lot of luggage, so he had to take a taxi.