This paper analyzes the adjective sentences with verbs in the infinitive and the contrast between these units in the Italian and Spanish languages. In first place there is the comparison between the adjective and relative sentences, and the possibility that the infinitive can result in a predicate sentence. The following reviews the principal Italian structures that are considered adjective sentences of relative with verb in the infinitive; on this basis the following corresponding sentences in Spanish are analyzed with the conclusion about the symmetry and asymmetry between the two languages and their potential consequences in the applications in the glotto-didactic contest.