Question

Where to use the modal verbs such as must, should, ought to etc? Can you explain with examples? What is the difference between them?

Answer 1

The modal verb "must" is the most imperative word of the three, followed by "ought to", then finally "should".

Explanation:

Must - important - has to be accomplished.
Ought to - something worthwhile or maybe necessary.
Should - depends on conditions.

To explain this idea in detail, I must use examples.

Must is used here because the sentence implies necessity.
I cannot explain properly without examples.

I ought to provide information on how they relate to the subject.

Ought to improves the power of this sentence by adding more force to the verb (over should).

If I use other sources of information, then I should cite those sources.

Should is used in this sentence to indicate a more conditional approach to the action as it is dependent on the "If".

You must pay the rent - it is due today.
You ought to pay the rent before the end of the month.
You should write monthly checks in advance to pay the rent.

Note that in most cases should will replace ought to in common usage, and is rarely used in questions, as the conditional is immediately implied in this type of question.