Question

Fill in the blank with a suitable modal:
If you need some help with your homework, you ________ go to the library.

• A. should
• B. mustn't
• C. had to
• D. shouldn't

Answer 1

The correct option is A should

Answer : A

Modal verbs are auxiliary verbs that are used to express necessity or possibility.

Let's take a look at the usage of the modal verbs in the options:

• should : It is used for expressing an opinion / advice / expectation / criticism about some past action or it can be used to ask for an advice
• mustn't : It is the contracted form of 'must not'; it is used to talk about things we need to avoid doing
• had to : It is used to express a necessity felt by the speaker in the past
  
• shouldn't : It is the contracted form of 'should not'; it is the negative form of the modal verb 'should'

Here in the question, we see that the speaker is expressing a suggestion, that is to visit the library in case he/she requires help.

It is important to note here that the sentence is conditional. And in particular, it is First Conditional Sentence -- we use 'should ' to express a small possibility in first conditional sentences.

• As it is a suggestion & not an obligation or something that needs to be avoided -- mustn't is not suitable here.
• The sentence is not in the past tense -- therefore 'had to' is not suitable here.
• The sentence is in the positive form -- therefore 'shouldn't' is not suitable here, also it would not be meaningful to tell someone they should not go to the library if they needed help with their homework, right?

Therefore from among the options, 'should' is the most suitable verb to complete the sentence.

Thus, the correct complete sentence is:

If you need some help with your homework, you SHOULD go to the library.