My sentence: 彼はこれを出来るなら僕は矢っ張り出来る
I can understand the rest of the changes, but why はー＞に ? All they said was that it sounded more natural but they weren't sure why. Could someone explain this to me? Thanks!
tl;dr: 「は」 is a big no-no for the subject marker in if-clauses.
A two-step explanation will be in order here.
First, there is a big grammar rule that says "DO NOT use 「は」 as the subject marker in an if-clause. Use 「が」 for that."
This seems to be a very common mistake among Japanese-learners. SE is no exception at all.
Secondly, 「できる」, which is used in your sentence, is a special verb that can take 「に」 with the subject as in 「(person) + に + できる」. However, the が-over-は rule for the if-clauses still stands, which is why it is incorrect to say:
which is essentially what you wrote.
That is because the first part of that sentence is an if-clause as it uses 「なら」. You just cannot use 「は」 there. Thus, it is only correct to say:
But as I mentioned above, it is more natural to use 「に」 than 「は/が」 with 「できる」. Thus, your teacher/friend corrected your sentence that used 「は」 to: