"I will break him."
Translation:אני אשבור אותו.
To anyone who does this exercise after me, I am a speaker of Hebrew at a native level who is passing through the levels of this course because I've been really excited for a long time about it coming out--that is why I feel confident pointing these things out.
It is not necessary to repeat the pronoun in this sentence: אני אשבור אותו
The conjugation, אשבור, already contains a subject marker which explicitly indicates that a first person singular speaker ("אני") is speaking. If Duolingo tells you that it was incorrect for you to submit the answer, אשבור אותו, without repeating the pronoun, then they're mistaken. In a longer sentence, possibly with a clause separating the pronoun from the verb, or in a more formal context, the pronoun might be retained. In this short a sentence, it is redundant. Including it might be perceived as unnatural.