Final answer:
The correct completion for the sentence 'If I were a boy, I ___ understand' is 'could,' as it reflects the hypothetical nature of the condition.
Step-by-step explanation:
If you were a boy, the correct sentence would be “If I were a boy, I could understand.” This sentence uses the conditional mood, which is used to express a hypothetical situation that is dependent on a condition being met. In this case, the condition is being a boy. The sentence does not align with past possibilities, which would use “could have,” and it’s not a confirmation of current ability, which would simply use “can.”