One doesn't, but there is a clear distinction between attempting to learn a language by memorizing its words and grammar, and learning it through context.
To actually understand a language in the same way we understand our native language, we cannot learn the language by memorizing the meanings of words and treating each sentence like a logic puzzle to be solved.
Practical knowledge of language is largely tacit.
A native speaker does not need to know what tense a word is in to use them, they do not even need to understand what a verb is. Explicit grammatical understanding can help us reason about a language, but it is not necessary to learn it.
