In the student's phrase "you must watch at the guy," what type of error is present?

The error in the phrase "you must watch at the guy" involves the incorrect use of a preposition. The phrase should instead use "watch" without a preposition, as the verb typically does not require a preposition when indicating the target of the observation. The correct phrase would be "you must watch the guy," where "the guy" directly follows the verb without any preposition in between.

Understanding the role of prepositions is essential, as they often indicate relationships in time, space, or direction. However, in this case, the verb "watch" operates directly on its object without requiring the additional preposition "at," making it a clear example of prepositional error in English usage. By recognizing this mistake, learners can improve their command of verb-object relationships in sentences.