Heritage grammars tend to undergo structural change owing to their severely constrained input conditions and /or transfer effects from the L2 (Polinsky, 2018). This study uses adjectives in Tamil (Dravidian) to show a systematic difference between rule-governed, structural aspects of grammar and those that require case-by-case lexical learning. The former remains stable and the latter undergoes change in heritage Tamil. The empirical domain of adjectives in Tamil is novel and particularly informative, as the derived nature of these adjectives helps us identify areas of grammatical stability and those of change when the context of acquisition diverges from the norm, i.e, heritage grammars. The paper has two major aims: (i) to provide an explanation of adjectives in standard Tamil, and (ii) to inquire into how the derivation of adjectives fares in the context of heritage Tamil. (i) is addressed by showing that adjectives in Tamil are not an independent category in the lexicon, but the derivational component recognises them as a distinct category. We then proceed to question (ii): With respect to heritage Tamil, two domains — one requiring intensive learning, and not requiring learning — are identified. The paper provides novel empirical evidence to demonstrate their stability or variation in heritage grammars.