Modals are a special grammatical category that are sometimes called "incomplete verbs". Modals always precede the base form of another verb and have special rules for negation and lack inflection. While modals follow specific grammar rules, modal-like expressions have the same meanings as modals but do not always follow the same rules, such as "have to", "supposed to", and "able to". Advice can be given using true modals like "should" or phrasal modals like "ought to".

1. Modals and Modal-like
Expressions

2. Modals have special grammar
characteristics. They are a special
grammatical category.
Some people call them “incomplete verbs.”
will, shall, would, should, can, could, must,
may, might

3. Modals have special grammar characteristics. They are
a special grammatical category.
A modal is always followed by the base form of
another verb. (Must go, can see, may have, etc.)
A modal has a special way to form a negative—may
not, may not have
Modals don’t change form: he must, not he musts

4. Modal-Like Expressions

5. A modal-like expression is an expression that has the
same meaning as one of the true modals.
Must = have to
Should = supposed to, ought to
Can = able to
A modal-like expression does not follow the grammar
rules of a true modal.

6. I must go =
I have to go
Must you go? =
Do you have to go?

7. Advice:
You should quit smoking (true modal) =
You ought to quit smoking (phrasal modal)