"What would you do if you were stronger?"
Translation:Wat zou je doen als je sterker was?
It can be jij for the first one. Since jij is stressed you point out the person with the first part of the sentence, then subsequently it's unnecessary and unnatural to also point the person out in the second part of the sentence. So grammatically it may not be wrong, but it sounds odd when you stress the person twice.
For audio exercises, the voice says je hence only je is accepted.
Ok, I think I got what you said... does this mean in speaking the Dutch would even leave out the "je" and simply say something like: Wat zou jij doen, als sterker was?" ... I'm only trying to clear up a little fog here. In English we would say, "What would you do if you were stronger". Is there something about the use of je that makes it less of a you than jij does? Is it a thing about "case"? I'm not trying to be a smart aleck, I'd really like to clear up the confusion. Bedankt in advance!