Second-language acquisition (SLA) is a genuine academic research field, and a handful of its core concepts explain a lot about why certain study approaches work better than others.
Linguist Stephen Krashen's input hypothesis argues that language acquisition happens mainly through exposure to input that's understandable but slightly beyond your current level — neither material you already fully understand, nor material so far beyond you that it's incomprehensible. This is part of why graded readers and beginner-friendly audio (matched to your actual level) tend to work better than diving straight into native-speed, native-complexity material.
Linguist Merrill Swain's output hypothesis pushes back on input alone being sufficient, arguing that actually producing the language — speaking or writing — forces you to notice gaps in your knowledge that passive listening or reading doesn't reveal. This is why practicing production (even simple, imperfect sentences) matters alongside consuming input, not just as an eventual final step.
Linguist Larry Selinker described "interlanguage" as the evolving, internally consistent (but not yet fully target-accurate) grammar system learners build on the way to fluency — a real, rule-governed system, not just a collection of random errors. This reframes mistakes as evidence of active progress through a structured system, not simply failures to be avoided (see Language Learning Myths).
The idea that there's a biological window after which language acquisition becomes much harder is real for certain aspects (particularly native-like accent), but weaker or contested for others (vocabulary, grammar, comprehension) — the strong pop-culture version ("you can't really learn a language as an adult") overstates what the actual research supports.
See also the reference page: Language Learning Myths.
A concept from linguist Stephen Krashen's input hypothesis: language acquisition happens primarily through exposure to material that's understandable but slightly beyond your current level, rather than material that's either too easy or too far beyond you.
A concept from linguist Merrill Swain arguing that actually producing language — speaking or writing — reveals gaps in your knowledge that passive input alone doesn't, making production practice an important complement to input.
A term from linguist Larry Selinker describing the evolving, internally consistent grammar system learners build on the way to fluency — a real, rule-governed system rather than a random collection of errors.
It's well-supported for some aspects, particularly accent, but weaker or contested for others like vocabulary and grammar — the popular version of the claim (that adults simply can't learn languages well) overstates the actual research.