Using data from simultaneous and sequential bilingualism, this paper addresses a series of learnability issues by investigating the acquisition of four different structures: (i) the [Gender] feature of the Spanish Determiner in the production of English/ Spanish mixed Determiner Phrases; (ii) the [Determiner] feature of Tense in the production of null/overt subjects; (iii) the morphosyntactic status of definite articles and clitics; and (iv) the morphosyntactic characteristics of Spanish deverbal compounds. The paper compares spontaneous and experimental data elicited from children and adults in order to show how these linguistic constructs can account for the differences and similarities between native and non-native bilingual systems. The data reveal clear-cut differences between native and non-native production/ interpretation. This is attributed to feature specification and the way in which both groups access the input.