Differences : Should, Must, Ought to and Have to

Today’s English
March 23rd, 2017

1.Should, Must and Ought to are modal auxiliaries expressing the mood but have to is not a modal auxiliary. 

2.Ought to is more formal but less common than should.  When something is morally right, or a law and rules to be followed, we generally use “ought to".  If it is just an instruction, advice or suggestion, we use “should".

You ought to take care of your parents in their old age. (Moral responsibility)

You ought to wear helmet to avoid penalties.(law to be observed)

You ought to follow the rules of the college, otherwise you will be suspended. (Rules to be followed)

You should come with your brother collect the money. (Just a common instructions or advice)

You should first speak in English that is the best way to improve your English. (Just an instruction or advice)

3.Must is a stronger form of should and ought to.  It makes something compulsory offering no choice. You know, Must is often an order but should is an advice. When you are very sure and make a conclusion, you can use must. We also use must to make strong recommendations.

You must come with your sister, otherwise I won’t give the money.
(Compulsion, stronger than should)

I’m sure, he must be her father. (Strong conclusion)

It’s a must-see movie / must-read book / must-have mobile.

4.Must is synonymous with “have to" in most of the contexts. However there are two differences: must expresses the speaker’s situation or wish but have to unfolds others' wishes or reactions.  Secondly, Must has no past tense or future tense like “have to" – have to/ has to, had to and will have to.

I must finish this work now itself. I have an urgent work tomorrow. (The speaker’s situation)
I have to finish this work now itself. Otherwise my daddy will hit the ceiling.

My wife is so suspicious. I have to delete this message. (Consequences will be severe)

He had no money. So he had to make a call. (Past tense)

We will have to achieve the target before Sunday as instructed.

(We can use must as a noun, can’t we?  First it is a must to the host just to provide best nest to the guest for rest.)