We investigate the acquisition of sentential complementation under causative, perception, and object control verbs in European Portuguese, a language rich in complement types, including the typologically marked inflected infinitives. We tested 58 children between 3 and 5 years of age and 24 adults on a sentence completion task. The results support two main hypotheses concerning children's initial biases in representing complement structure. The first pertains to argument structure--a verb selects only one internal (propositional) argument ("Single Argument Selection Hypothesis"), the other to syntactic structure--propositional complements are complete functional complements ("Complete Functional Complement Hypothesis"). These initial biases lead children to avoid raising-to-object and object control structures, in favor of finite complements and inflected infinitive complements, the latter appearing in both target and nontarget contexts.