Question

Fill in the blank with a suitable modal:
I ________ swim when I was three.

• A. could
• B. can
• C. have to
• D. must

Answer 1

Answer: (A)

could

Answer : A

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

• could : It is used to express an ability or a lack of an ability / make a polite request / make a suggestion / to express specific possibility / make a negative deduction; important to note is "could" is used in Past Tense

• can : It is used to express an ability or lack of an ability / to give permission or express a prohibition / to make an informal request / to express a general possibility (only with positive verb) / to make a negative deduction; important to note is that "can" is used in Present & Future tense
• have to : It is used to express an external obligation in positive sentences and removing an external obligation (or necessity) in the negative sentences
• must : to express a necessity (in the present or future); to make an assumption that something is true

Here in the question, we see that the speaker is expressing about his / her ability to swim when he / she was three years old.

• As the sentence is in Past Tense, "can" is NOT appropriate.
• "have to" is NOT used to express an ability, thus it is also NOT appropriate.
• "must" is also NOT used to express an ability, thus it also is NOT appropriate.

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

Thus, the correct complete sentence is:

I COULD swim when I was three.