Second language acquisition (SLA) studies how people learn additional languages after their first, a process that differs fundamentally from child L1 acquisition. The critical period hypothesis suggests that full native-like attainment becomes increasingly difficult after puberty, particularly for phonology and morphosyntax. Learners construct an interlanguage — a systematic, evolving grammar that is neither their L1 nor the target L2 — shaped by L1 transfer, overgeneralization, and developmental sequences. Adult learners bring metalinguistic awareness and literacy skills that children lack, but they also face interference from entrenched L1 patterns and typically require explicit instruction for features that children acquire implicitly.
Analyze interlanguage samples from learners of different L1 backgrounds acquiring the same L2, identifying which errors reflect transfer and which are developmental. Compare your own L2 learning experience against the theoretical predictions — notice where your L1 helps and where it hinders. Read key studies on the critical period (Lenneberg, Johnson & Newport) and evaluate the evidence for and against an age cutoff.
From your study of first language acquisition, you know that children acquire their L1 effortlessly, rapidly, and without formal instruction — driven by rich exposure and a highly plastic language faculty. Second language acquisition (SLA) asks: what happens when a post-childhood learner tries to add another language? The answer is not simply "repeat the first acquisition process later." The mechanisms, the outcomes, and the challenges are fundamentally different, and understanding why tells you as much about language as about learning.
The most important concept in SLA is interlanguage — the term linguist Larry Selinker coined for the systematic grammar that learners construct as they progress toward the target language. An interlanguage is not simply "broken L2." It is a real, rule-governed grammar — just one that differs from both the learner's L1 and the target L2. Learners at different stages have different interlanguages. A Spanish speaker learning English might consistently produce "She no go to school" — which violates both Spanish (Spanish uses no before the verb differently) and English (standard negation is doesn't go). This is an interlanguage rule, not random error. Interlanguages evolve as learners get more input and feedback, but they can also fossilize — stabilize at a point short of target-language norms, particularly for features that do not interfere with communication.
Two forces shape interlanguage development. L1 transfer occurs when the learner applies a rule from their first language to the second. If the L1 and L2 share a feature, transfer helps — a French speaker learning Spanish already knows verb conjugation. If they differ, negative transfer produces systematic errors: a Japanese speaker learning English tends to drop articles because Japanese has none, and a German speaker may place verbs at the end of English clauses by analogy with subordinate clauses in German. The other force is developmental sequence: regardless of what L1 learners have, they all tend to acquire certain L2 features in roughly the same order. English negation, for example, follows a predictable sequence (no + VP → don't + VP → doesn't/didn't + VP) that holds across learners of different L1 backgrounds. This implies some aspects of acquisition are driven by universal cognitive strategies rather than L1 habits.
The critical period hypothesis holds that full native-like attainment in an L2 becomes progressively harder after early childhood, particularly for phonology and morphosyntax. The neurological evidence suggests the language-learning system's plasticity declines gradually rather than dropping off a cliff at puberty. Adult learners compensate with genuine advantages: metalinguistic awareness (you can reflect on grammar rules explicitly), literacy, world knowledge, and motivation. These make explicit instruction more effective for adults than for children. But adult learners face a disadvantage that children never encounter: an entrenched L1 grammar that exerts constant pressure on the developing interlanguage. Managing that pressure — noticing where your L1 is distorting your L2 output, seeking corrective feedback, and deliberately targeting fossilized features — is the core skill of successful adult language learning.