This dissertation is in the context of a theory which reconciles a psycholinguist approach to language acquisition and a minimalist conception of language. The working hypotheses were: grammatically relevant information has to be legible at the phonetic interface; the semantic interpretation of the TAM complex (Tense, Aspect, Mood) poses difficulty to the identification of the verbal mood morphology; basic conceptual distinctions, such as the realis/irrealis contrast, are expressed by the child, no matter whether or not his/her language morphology has been totally identified (The Semantic Opposition Hypothesis). A study of the spontaneous speech production of two children, aged 1;5 to 2;5, and 4 experiments were carried out. The analysis of the production data revealed that the realis/irrealis opposition is expressed by the child by means of the distinction between inflected/uninflected verbal forms and the Root Infinitive (RI) represents an alternative form to the subjunctive for the expression of the irrealis mood. The results of two elicited production experiments suggest that the indicative/subjunctive distinction is perceived by 3-year-olds, although the expression of the subjunctive mood still poses difficulty even for 5-year-olds. A comprehension experiment involving YES/NO questions with 5-7-year-olds, and its follow-up, suggest that the indicative/subjunctive opposition in subordinate clauses is recognized, even though negative answers pose difficulties that are not related to verbal mood, particularly at 5 years of age. The results are compatible with the hypotheses, suggesting that the presence of RIs in Brazilian Portuguese indicates a parametric shift in course (from null subject to non-null subject).