The sentence is grammatically incorrect because it lacks a proper structure. The phrase 'celui que Monique' is incomplete and should be followed by a verb or a complement to clarify what is being referred to. A correct version could be 'celui que Monique aime' to indicate that Monique likes a specific book. Additionally, the verb 'aimes' is correctly conjugated for the subject 'tu', so the error lies in the incomplete phrase rather than the verb conjugation.