If you don't speak a language, you can buy a grammar, or ask native speakers to think up some examples and build rules from them.

You do that when you're a complete beginner, or to polish off your grammar to avoid coming off as uneducated esp. in writing, but the way you actually learn a language well enough to have a conversation without too many misunderstandings by either party is by listening to it (and, when you get a chance, speaking it) a lot. And many of the things you'll learn this way are things that few grammars will explicitly state and few native speakers will admit. No amount of theoretical study will train your ear to understand speech in real time. You cannot rely on System 2 alone to speak a natural language, as per Moravec's paradox.

The analogy would be that you learn body language by paying attention to what people who already know it. More generally, ISTM that paying attention to stuff around you (and also paying attention to what you are doing, for that matter) is an oft-neglected skill. (Dear myself-a-few-years-ago, are you listening?)