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Modal Auxiliary Expressions:
should, ought to, must, have to

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Modal auxiliary expressions are words or phrases that change the meaning of a verb.
Modals are used directly before the verb in a sentence.
Always use the base form of the verb following a modal auxiliary:

Present or  Base form	Past form	Present Participle	Past Participle
ask	asked	asking	asked
need	needed	needing	needed
scrape	scraped	scraping	scraped
run	ran	running	ran
give	gave	giving	given
go	went	going	gone

Here are some common modal auxiliaries:

should

ought to

must

have to

should, ought to

Should and ought to have the same meaning.

Violeta should learn some English before she visits her sister in Chicago.

Violeta ought to learn some English before she visits her sister in Chicago.

In the present, they both mean advisability.
In the future, they both mean predictability.

should	present	You should have a flu shot if you go to the U.S. in January.	It is advisable...it is the smart thing to do.
	future	You should be safe from the flu if you get a flu shot before you go.	This is something that we can predict. It's 90% certain.
ought to	present	You ought to bring warm clothes if you visit the U.S. in the winter.	It is advisable...it is the smart thing to do.
	future	The temperatures ought to be below zero Celsius for most of January and February in the northern states.	This is something that we can predict. It's 90% certain.

shouldn't
In the present, shouldn't means inadvisability.
In the future, shouldn't means predictability of a negative outcome.

present	You shouldn't bring food when you visit the U.S.	This is inadvisable. It may not be allowed.
future	You shouldn't have a problem with canned food.	This is something we can predict. It's 90% certain.

When native speakers use should not in its uncontracted form, it is for emphasis and sometimes to express extreme inadvisability.

You should not joke with U.S. customs officials!

Ought not is uncommon in the present and in the future. It is rarely contracted.

must, have to

Must and have to both mean necessity.
They also mean 95% certainty as a conclusion.

must	You must have a valid passport to travel to the U.S.	This is required by law.
	Maria's brother has been in the U.S. all his life. He must speak very good English.	This is a 95% certainty. This is a conclusion that can be drawn.
have to	You have to get permission to study in the U.S.	This is required by law.
	I checked my addition several times. The amount has to be correct.	This is a 95% certainty. This is a conclusion that can be drawn.

must not, don't have to
Must not means prohibition. It is never contracted.
Don't have to means lack of necessity. It requires a form of do.

must not	You must not bring plants into the United States without permission.	That is forbidden, prohibited.
	You must not bring guns or explosives into the U.S.	That is forbidden, prohibited.
don't have to	You don't have to speak English in some neighborhoods in the U.S. Some people live here for years without learning English.	It's not necessary.
	You don't have to be a citizen of the United States to register in a college.	It’s not necessary.

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