Most important for what? For speaking about linguistics, maybe. But if you are using the language to communicate, there is no most important single thing to "learn", since using the language to communicate means that you are using the language in a way that resembles a skill, and the language grows in your brain as an implicit system – you acquire it bit-by-bit, but the internal system doing the acquiring doesn't care what counts as "grammar" and what counts as "lexicon". You can teach yourself rules, but that doesn't mean that you are able to use them spontaneously.

TL;DR: knowledge about language is different from skill of communicating in language.