Question

Fill in the blank with a suitable modal:
You _______ tell lies. It's wrong.

• A. need to
• B. should
• C. have to
• D. shouldn't

Answer 1

Answer: (D)

shouldn't

Answer : D

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

• need to : It can be used instead of 'must' & 'have to' IF the distinction between the 'speaker's opinion' & 'external obligation' is unimportant
  
• 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
• have to : It is used to express an external obligation in positive sentences and removing an external obligation (or necessity) in the negative sentences
• 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 an opinion, that is to NOT tell lies because it's wrong.

• 'need to' is not suitable here because it would express an opinion in the positive form; and it would not be meaningful.
• 'should' again would imply a positive opinion, which is not opinion being conveyed here by the speaker
• 'have to' implies an external obligation in positive sentences, which again would not be suitable here, as it would be contrary to the opinion that the speaker is trying to convey here.

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

Thus, the correct complete sentence is:

You SHOULDN'T tell lies. It's wrong.