Learn Farsi: به گفته‌ی مجله، تو می‌توانستی استایل خود را تغییر دهی. - According to the magazine, you could change your style.

The phrase به گفته (be gofte) literally means 'to the saying' or 'in the words of'. It is used as a prepositional phrase of attribution. Because گفته ends in a silent 'heh' (ه), you must use a 'ye' (ی) for the ezāfe to link it to the noun مجله (magazine), giving us به گفته‌ی مجله (be gofte-ye majalle).

The translation says 'you could change' (past tense). Why is the second verb تغییر دهی (taghyir dehi) in the present subjunctive?

When using the modal verb توانستن (tavānestan - to be able to), it is conjugated to show the tense, while the main action verb that follows takes the present subjunctive form. Here, می‌توانستی (mi-tavānesti) shows the past ability ('you were able to'), and the dependent verb remains present subjunctive regardless of the past context.

If تغییر دهی is subjunctive, why doesn't it have the usual بـ (be-) prefix like تغییر بدهی?

In written or formal Farsi, the subjunctive بـ (be-) prefix is often dropped in compound verbs, particularly those made with دادن (dādan) and کردن (kardan). Both are correct, but dropping the prefix fits the slightly formal, journalistic tone of this sentence.

For 'your style', why does the sentence use استایل خود را instead of simply attaching a pronoun like استایلت را?

While استایلت را (estāyl-at rā) is perfectly correct and very common in conversation, written Farsi frequently uses the reflexive pronoun (khod - self/own) instead of attached pronouns. It sounds slightly more formal and emphasizes 'your own style'.