Traditionally taken as one of the nominals forms of the verb, infinitives in Portuguese, in spite of their morphophonological homogeneity (-r), appear in a range of morphosyntactic contexts. This paper argues that all these contexts can be reduced to three, namely, a nominal, a verbal and a “mixed” – in the sense of Chomsky (1970). Specifically, by assuming Distributed Morphology (Halle & Marantz, 1993), it proposes a step-by-step morphosyntactic and semantic derivation of the contexts where the /R/-morpheme appears, also addressing the issues involving tense and aspect in these forms. In summary, we argue that nominal, verbal and mixed infinitives share the same derivational path up to AspP, which – we claim – is underdetermined, thus competing with (perfective) participles and (imperfective) gerunds. Above AspP, the first bifurcation is between no and T. Projecting no generates a predicate of events. By projecting T (in verbal and mixed infinitives), the system has a three-way path: T[+tense], where T will be futurate in relation to the finite matrix verb; T[–tense], where T will be simultaneous /anaphoric in respect to the finite matrix verb; or T[ ], where there is no c-commanding T and then, as last resort, the grammar will change this phrase into a predicate of events.