I made the same mistake, I'm no language expert but I think it may have something to do with which objects play the role of "direct" and "indirect" object. "I rented this car when I was you" the car is the direct object here and the second I is the indirect object. In the given translation the roles are reversed. Just a guess maybe someone that knows more can confirm or refute that.
In the last sentence I wrote: "He did not talk with the guard when he arrived at the bakery's garden" and I had been corrected to "When he arrived at the bakery's garden he did not talk with the guard". Therefore now I wrote: "When I rented this car I was Young" and now Duolingo corrected me to "I was young when I rented this car"
Are that only forgotton alternatives or is there really a differeny if I start with the subordinary clause or end with it????? Please let me know.