Pembahasan
Step 1: find the main verb — “忘在” (leave behind at). Because of “把”, this is a 把-sentence: subject + 把 + object + verb + what follows. Step 2: who forgot? Only “他” is a person, so “他” opens the sentence, followed by “把手机” (the thing left behind). Step 3: “忘在” needs a place after it, which is “出租车上了”; the final “了” closes the sentence. Answer: “他 + 把手机 + 忘在 + 出租车上了”. A common mistake for Vietnamese learners is putting the place before the verb (“他把手机在出租车上忘了”): with “V在 + place” the place must follow the verb.