One of the intricate properties of second language acquisition is that some types of grammatical feature appear to be acquired easily and in a native-like way, while others remain persistently difficult. There is much evidence related to the issue in the current empirical studies, and the issue raises an important explanatory problem for a theory of second language acquisition. In this paper, an attempt is made to shed light on the point whether the variability in morpho-syntactic features in adults' L2 production is due to a competence or a performance effect. On the basis of the evidence gained through the literature in the field, it is concluded that both competence and performance effects are involved in the optionality of the use of inflectional morphology. This paper also reveals that through the development of interlanguage, there is nothing as impairment, whether local or global, to hinder the correct grammatical representation of underlying syntax; morphology will be learned, and morphological paradigms are gradually added to the lexicon, in the same way as words do.