This article analyses how syntactic and semantic dimensions come into play in a grammatical activity conducted in primary school classes in French-speaking Switzerland. The data come from an SNSF research, the didactic engineering part of which involved developing and testing teaching sequences on the noun complement. The analysed task requires grammatical reasoning in order to decide on the presence or absence of this grammatical function in pairs of sentences, while encouraging the justification of the given answer. We focus on two operations: identifying the noun complement and differentiating it from other syntactic functions. The analyses show the role played in this context by syntactic and semantic arguments, their advantages and limitations, and their effects on the understanding of the grammatical notion in question. The results of the analyses draw attention to the potential pitfalls of both a mechanical syntactic approach and a semantic approach, and above all to the importance of not confusing these two orders of dimensions.