Modals are types of auxiliary verbs that show a speaker's attitude about whatever they are expressing.
can, could, may, might, must, shall, should, will, would
This site has some exercises if you would like to test yourself.
Since they are forms of helping verbs, they must always be used with a main verb (as part of a verb phrase). They are never to be used by themselves.
I could play the piano for you.
Could is being used with the main verb play.
Will is being used with the main verb follow.
These are not, "marked for tense." That means that they don't change form based on the tense of the main verb.
They will never end in -ed to show past tense or -ing to show a present participle.
There is also no third person singular form. That means that they will never end in -s.
Subject Verb Agreement
These guys don't follow subject verb agreement.
Jonathan can rides the bus.
That sentence is all wrong, but it seems like it should be right!
The singular subject Jonathan means that our main verb rides should end in s. But, that rule does not apply here!
Jonathan can ride the bus.
That's better! There should be no -s after ride.
Making Modals Negative
In order to make these negative, you need to add not or another negative between the modal and the rest of the verb.
In these examples, I have underlined the whole verb phrase. Note that the word not is not part of the verb phrase.
I should not watch this movie.
Not is making this statement negative.
You could also make it a contraction, blending the words should and not together.
I shouldn't watch this movie.
When forming a question, the modal verb comes before the subject, and the main verb comes after the subject.
Will you take me to the park?
Should Jeremy come with us?
Are you interested in sentence diagramming? You should be. It will knock your socks off.