Question

Fill in the blank with a suitable modal:
It's my mother's birthday tomorrow. I ________ buy her a present.

• A. must
• B. used to
  
• C. shouldn't
• D. don't have to

Answer 1

The correct option is A must

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:

• must : to express a necessity (in the present or future); to make an assumption that something is true
• used to : It is used to refer to some activity that took place in the past in a habitual way
  
• shouldn't : It is the contracted form of 'should not'; it is the negative form of the modal verb 'should'
• don't have to : It is the negative form of 'have to'; it is used to remove necessity or external obligation

Here in the question, we see that the speaker is expressing a necessity felt by him / her, that is to my a present for her mother as it is her birthday.

• 'used to' is not a modal verb, moreover the sentence is not in past tense, which makes its 'used to' unsuitable here.
• 'shouldn't' is not suitable because the speaker is expressing a necessity felt by him / here and not an advice; moreover the sentence is in positive form, & 'shouldn't' is used with negative sentence.
• 'don't have to' eliminates the necessity instead of expressing it.

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

Thus, the correct complete sentence is:

It's my mother's birthday tomorrow. I MUST buy her a present.