Error-spotting questions split a sentence into parts and ask which contains the mistake. Scan in a fixed order: subject-verb agreement, tense, preposition, article, then word choice. A fixed order stops you from missing the obvious.
Voice conversion appears regularly. "She writes a letter" becomes "A letter is written by her": the object moves to the front, the verb takes the past participle, and the subject follows "by".
Redundancy is the last check. "Return back", "repeat again" and "more better" are all incorrect because the extra word adds nothing.