Learning and Teaching Second Languages

Second language acquisition — usually shortened to SLA — is both a thing that happens and the field that studies it. As a process, it is the way a person comes to know and use a language other than the one they grew up with, whether in a classroom, on the street of a new country, or through a phone app. As a field, it is the branch of applied linguistics that asks how that learning actually works: what stages learners pass through, why the process so often stalls before native-like mastery, and why two people given the same lessons end up so far apart. This entry is the map of that field — a hub from which the more detailed topics on this site branch out.

The term "second" is used loosely. It covers a third or fourth language just as well as a second, and it does not distinguish learning a language while immersed in a country where it is spoken from studying it as a "foreign" language in a classroom back home. What all these cases share is that the learner already has at least one language in place — and that first language, together with the learner's age, motivation and aptitude, shapes everything that follows.

SLA as a field: errors and interlanguage

SLA became a research field of its own in the late 1960s and early 1970s, and it did so by taking a fresh look at something teachers had always treated as a nuisance: learners' mistakes. In a short and hugely influential 1967 paper, "The Significance of Learners' Errors", the British linguist S. Pit Corder argued that errors are not simply failures to be stamped out. They are evidence. A consistent mistake shows that the learner is operating a rule — the wrong rule, perhaps, but a rule — and so it opens a window onto the internal grammar the learner is building. This shifted the study of errors from correction to error analysis: reading mistakes as data about how the mind constructs a new language.

Five years later, the American linguist Larry Selinker gave the idea its central concept. In a 1972 paper he coined the word interlanguage to describe the learner's language as a system in its own right — not broken English or half-learned French, but a coherent, rule-governed grammar that sits between the first language and the target and evolves as learning proceeds. Interlanguage is systematic (its "errors" follow patterns), it is dynamic (it changes over time), and it is shaped partly by transfer from the first language and partly by the learner's own hypotheses. Selinker also named a phenomenon every teacher recognises: fossilisation, where some features of the interlanguage stop developing and settle permanently short of the target, however much input the learner receives.

One of the field's earliest and most durable findings is that this development is not random. Studies of learners acquiring English — beginning with Heidi Dulay and Marina Burt's "morpheme studies" in the 1970s — found that certain grammatical features are acquired in a broadly predictable order, and that this order is largely the same regardless of the learner's first language and only weakly affected by the order in which things are taught. A learner masters the "-ing" of the present continuous early, for instance, and the third-person "-s" much later, whatever the syllabus says. This "natural order" suggests that acquisition follows an internal timetable of its own, one that instruction can support but not simply overwrite.

The main hypotheses: input, interaction, output

If learners build an interlanguage on an internal schedule, the obvious next question is what feeds that process. Three answers, proposed in turn, have organised much of the field's debate — and they are complements more than rivals.

The input hypothesis came from Stephen Krashen around 1980. Its claim is that language is acquired in one way only: by understanding messages slightly beyond one's current level — comprehensible input, captured in his formula i+1. On this view, understanding comes first and grammar is picked up along the way, so the learner's main job is to get large amounts of input they can mostly follow. Comprehensible input is now almost universally accepted as necessary for acquisition; the argument has been over whether it is sufficient.

The interaction hypothesis, developed by Michael Long in the 1980s and 1990s, answered that input matters most when it is negotiated. When two speakers do not quite understand each other, they repair the conversation — repeating, rephrasing, checking, clarifying — and this "negotiation of meaning" makes input more comprehensible and draws the learner's attention exactly to the points where their interlanguage falls short. Interaction, in other words, turns raw exposure into usable input.

The output hypothesis came from Merrill Swain in 1985, out of a puzzle in Canadian French immersion classrooms. Students there had received years of rich, comprehensible input and understood almost like native speakers — yet their spoken grammar stayed stubbornly inaccurate. Swain concluded that input alone does not build production: learners also have to be pushed to produce language, because speaking and writing force them to process grammar actively, notice the gaps in what they can say, and test hypotheses in a way that listening never demands. Together the three hypotheses give a rounded picture — understand a great deal, interact to sharpen that understanding, and produce in order to turn it into fluent, accurate use.

What makes a learner succeed: age, aptitude, motivation

The hypotheses describe the mechanism of acquisition; they do not explain why outcomes vary so widely. Three learner factors account for much of that variation.

Age. The observation that children usually reach a higher final command of a second language than adults is often explained by the critical period hypothesis, proposed for language by Eric Lenneberg in 1967: a biologically favoured window, closing around puberty, within which the brain acquires language most readily — especially accent. The picture is not that adults cannot learn (they often learn faster at first, drawing on mature memory and reasoning) but that native-like ultimate attainment, particularly in pronunciation, becomes rare after that window closes.

Aptitude. Even among learners of the same age, some are simply quicker. Language aptitude — measured historically by tests such as John Carroll's Modern Language Aptitude Test — is not a single talent but a cluster: an ear for sounds, a feel for grammatical patterns, and a strong memory for verbal material. Aptitude predicts how fast someone progresses more than whether they can succeed at all, and different components matter for different tasks.

Motivation. Of all the individual factors, motivation is the one most under the learner's own control, and it is a powerful predictor of success. Robert Gardner's classic distinction separated integrative motivation (a wish to connect with the people and culture of the language) from instrumental motivation (a practical goal such as a job or an exam); later work by Zoltán Dörnyei reframed it around the learner's future self-image and the day-to-day effort of staying engaged. Sustained motivation is what keeps a learner supplying themselves with input and practice over the years that acquisition actually takes.

Acquisition versus learning

Running underneath all of this is a distinction that has shaped the field since Krashen made it central: the difference between acquisition and learning. Acquisition is the subconscious process by which we absorb a language through understanding and using it, the way children pick up their first language — it produces the intuitive "feel" for what sounds right and drives fluent, unmonitored speech. Learning is the conscious study of the language as an object: rules, paradigms, vocabulary lists, deliberate error-correction. In Krashen's strong version the two are separate stores, and only acquisition produces spontaneous use, with conscious learning acting merely as a "monitor" that edits output when there is time to think.

Few researchers today accept that acquisition and learning are wholly walled off from each other — the balance of evidence is that conscious study can feed fluent use, especially for adults, and that explicit and implicit learning interact. But the distinction remains useful because it names two genuinely different kinds of knowledge: the rule you can recite and the rule you actually use under the pressure of real conversation. Much of the practical craft of teaching and learning a second language is about converting the first kind into the second.

What this means for learning a language

Decades of SLA research do not reduce to a single method, but they do point in a consistent direction, and a few conclusions carry over directly to anyone studying a language now.

Get large amounts of input you can mostly understand. The one point on which almost everyone agrees is that comprehensible input is the raw material of acquisition. Choose material a little above your level, on topics you already know something about, and consume a lot of it.
Do not stop at understanding — produce and interact. Swain's immersion learners understood but could not speak accurately. Building output and conversation in from early on is what turns comprehension into usable skill.
Expect an internal timetable. Because grammar is acquired in a natural order, some structures will resist being taught before you are ready for them. Steady exposure over time does what a single rule-explanation cannot.
Play to what you can control. You cannot change your age or reset your aptitude, and the critical period is real — but motivation and consistent practice are yours to manage, and they matter more over the long haul than any head start.

A learning tool is really a bet on how these findings fit together. The Taalhammer method is built to cover both halves of the input–output debate at once — massive comprehensible input paired with production from the first day — so that understanding and speaking grow together rather than one waiting on the other.

FAQ

What is second language acquisition in simple terms?

It is the process of learning a language other than your first — and the academic field that studies that process. SLA research looks at how learners build up a new language step by step, why they make the errors they do, and why factors like age, motivation and aptitude make such a difference to the result.

What is interlanguage?

Interlanguage, a term coined by Larry Selinker in 1972, is the learner's own version of the target language at any given moment: a systematic, rule-governed grammar that sits between the first language and the target and evolves as learning goes on. Its "errors" are usually not random but follow patterns, which is why they reveal how the learner's internal system works.

Is comprehensible input enough to learn a second language?

Most researchers say it is necessary but not sufficient. Understanding messages in the language (comprehensible input) is agreed to be the foundation of acquisition, but studies of immersion learners show that input alone leaves speaking inaccurate. Interaction and pushed output — actually using the language — are needed as well, and for adults some conscious study helps too.

Sources

Corder, S. P. (1967). The Significance of Learners' Errors. International Review of Applied Linguistics, 5(4), 161–170.
Selinker, L. (1972). Interlanguage. International Review of Applied Linguistics, 10(3), 209–231.
Krashen, S. D. (1982). Principles and Practice in Second Language Acquisition. Oxford: Pergamon Press.
Long, M. H. (1996). The role of the linguistic environment in second language acquisition. In W. Ritchie & T. Bhatia (Eds.), Handbook of Second Language Acquisition (pp. 413–468). San Diego: Academic Press.
Swain, M. (1985). Communicative competence: Some roles of comprehensible input and comprehensible output in its development. In S. Gass & C. Madden (Eds.), Input in Second Language Acquisition (pp. 235–253). Rowley, MA: Newbury House.
Lenneberg, E. H. (1967). Biological Foundations of Language. New York: Wiley — the critical period hypothesis.
Dörnyei, Z. (2005). The Psychology of the Language Learner: Individual Differences in Second Language Acquisition. Mahwah, NJ: Lawrence Erlbaum.