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What is the difference between the following sentences.
He was wrestling when he hurt his ankle.
He was wrestling when hurt his ankle.
Is it okay not to repeat the same subject, he in this case, after the word when or is it necessary?
Which one is more idiomatic?
The word when can take finite clauses as a Complement (clauses with a tensed verb). When we have a finite clause, the Subject of that clause must be present:
This is not true about non-finite clauses, which do not have any tense:
In the sentence above the non-finite verb being does not need a Subject. It has no tense, it is a participle, not a tensed verb. We understand the Subject of being to be the same as the Subject of the verb in the main clause, Bob.
The Original Poster's example
The conjunctive preposition when can take clauses with tensed verbs. When these clauses have tensed verbs (past or present tense), they must have their own Subject.
Although the Subject of hurt is the same as the Subject of was wrestling, we need to use a full Subject for the second clause:
He was wrestling when he hurt his ankle.
He was wrestling when hurt his ankle. (ungrammatical)
If you leave out the second pronoun, you could write it like this:
He was wrestling and hurt his ankle.
dictionary.com includes this definition of "when":
upon or after which; and then:
We had just fallen asleep when the bell rang.
It needs a main clause after it because it is, in a way, joining two sentences. "This happened" when "this happened". Without the pronoun, "hurt his ankle" is just a fragment of a sentence.
