Question

when he was young, he......... swim very well.​

Answer 1

The answer is: “when he was young he could swim very well”.

The sentence has two parts, which are the two clause of the sentence.

According to the English grammar rule, a sentence must be written with consistency of tense.

In the first part of the sentence the past tense “was” is used and that is why in the second part of the sentence could will be used.  

Answer 2

could

hope it will help you.......