Interlanguage in ESL Learning: Understanding Its Impact and Evolution

When a student writes “He go to school every day” or says “I am having three brothers,” they are not simply making careless mistakes. They are doing something far more systematic – they are operating in an interlanguage, a transitional linguistic system that sits between their first language (L1) and the English they are trying to learn. Understanding interlanguage is one of the most practical things an English teacher can do, because it reframes how we read student errors and how we respond to them.

What is interlanguage? 🔗

The interlanguage hypothesis, introduced by American linguist Larry Selinker in his landmark 1972 paper, describes the language system that second-language learners produce as they work toward target-language proficiency. Selinker coined the term by adding the Latin prefix inter- – meaning “between” or “among” – to signal that this system falls between the learner’s native language and the language being learned. It is neither one nor the other; it is a system in its own right.

Before Selinker’s work, two researchers had touched on the same idea independently: William Nemser called it an approximative system, and Pit Corder termed it transitional competence. What Selinker did was formalize and name it, and argue that learner language is governed by systematic rules – rules that differ from both the native language and the target language. At every stage of learning, students are not merely copying native speakers; they are constructing an entirely new language system unique to themselves.

This was a radical shift. Prior to interlanguage theory, the dominant behaviorist view treated learner errors as bad habits that had to be broken through drills and rote correction. Selinker’s framework, by contrast, showed that errors are evidence of a learner’s developing system – logical, predictable, and even necessary steps on the road to acquisition.

The three main causes of interlanguage 🔗

Selinker identified five central processes that shape a learner’s interlanguage: L1 language transfer, transfer of training, learning strategies, communication strategies, and overgeneralization of target-language rules. In practice, three of these are most visible in the classroom.

1. First language (L1) transfer 🔗

This is the most well-documented cause. L1 transfer is the influence resulting from similarities and differences between the target language and any language the learner has previously acquired. When a Hindi or Bengali speaker says “He is not knowing the answer,” they are applying an L1 grammatical logic – using the progressive form with stative verbs – that does not hold in standard English. Transfer is not always problematic; when L1 and L2 share structural similarities, the result is positive transfer that speeds up learning. But where the two systems diverge, negative transfer (or interference) causes persistent errors.

Selinker described this as “interlingual identification” – learners perceive certain units as equivalent across their L1, their interlanguage, and the target language, and apply them accordingly, even when the equivalence breaks down. A Spanish learner of English, for instance, may transfer the word “table” and apply it in expressions like “table the motion,” not realizing the English word carries meanings that have no parallel in Spanish.

2. Overgeneralization of target language rules 🔗

Once a learner grasps a rule in the target language, they often apply it too broadly – beyond its actual scope. A classic example is the English past tense: a learner learns that “-ed” signals past action and starts producing forms like *drinked, *goed, and *teached, treating irregular verbs as if they follow the regular pattern. Similarly, omitting the third-person “-s” in “she eat rice every day” is a case where the learner simplifies the rule to avoid the additional burden of tracking agreement marking.

This process is also seen in child language acquisition, which suggests it is not a failure of learning but a natural cognitive strategy – the learner is actively constructing grammar, not passively absorbing it.

3. Transfer of training and communication strategies 🔗

Sometimes the way a language is taught itself introduces errors. If a textbook or teacher consistently reinforces a simplified rule – for example, presenting “going to” as always interchangeable with “will” – learners internalize that simplification. Transfer of training refers to exactly this: mistakes that arise not from the learner’s L1 or from the target language itself, but from the instruction they received.

Communication strategies also shape interlanguage. When learners lack the vocabulary or grammar to express something precisely, they resort to paraphrase, simplification, or code-switching. These workarounds, while useful in the short term, can harden into habitual patterns if left unchecked.

Fossilization: when interlanguage stops evolving 🔗

One of the most significant effects of interlanguage – and one of the most challenging for teachers – is fossilization. Fossilization occurs when an interlanguage ceases to develop, locking certain non-target forms in place permanently or for a prolonged period. It can happen at any stage of learning and can affect motivated, intelligent learners who receive adequate instruction and exposure.

Selinker observed that approximately 95% of adult L2 learners never reach full native-level competence – not because they lack effort, but because fossilization intervenes. A learner who successfully communicates their meaning with their current proficiency level has less pressure to refine form and structure, and so the interlanguage stabilizes rather than advances.

Fossilization can be temporary (also called stabilization), where development stalls for a period before resuming, or permanent, where certain deviant forms become an unchangeable part of the learner’s production. It occurs across all levels of language – phonology, grammar, vocabulary, and pragmatics – and its causes include L1 interference, overgeneralization, inadequate corrective feedback, and classroom environments that prioritize fluency at the expense of accuracy.

The dynamic and variable nature of interlanguage 🔗

Interlanguage is not static; it shifts depending on context. A learner may produce a target-like form such as “I don’t” in a monitored writing task but revert to “me no” in casual conversation. This variability is a core feature of interlanguage, not an anomaly. It shows that learners’ grammatical knowledge is applied selectively, with more accuracy in planned, form-focused situations and more deviation in spontaneous speech where cognitive load is high.

This also means that interlanguage is permeable – it can change, be restructured, and move closer to the target language with the right kind of input and feedback. Researchers like Pienemann have shown that learner interlanguages develop along relatively predictable paths, and that instruction pitched one step above a learner’s current level is most effective in pushing that development forward.

Implications for English language teaching 🔗

Understanding interlanguage shifts the teacher’s role from error-hunter to diagnostic practitioner. Here is what the research suggests for classroom practice.

Treat errors as evidence, not failure 🔗

Errors are a reflection of the learner’s temporary language system and a natural part of the learning process. Recognizing this opened the door to communicative teaching methods, group work, and pair work – activities that would have seemed counterproductive when errors were considered purely harmful habits to be suppressed. A teacher who sees “she go” not as laziness but as a visible stage of interlanguage development can respond with far more precision.

Use targeted and timely corrective feedback 🔗

Research on corrective feedback shows that recasts – where the teacher reformulates a learner’s incorrect utterance correctly without explicitly flagging it as an error – are more effective than simply presenting a grammatical model. Interaction enhancement, where learners are guided to notice a mismatch between their interlanguage and the target form, and then prompted to self-correct, is also well-supported. Learners in feedback-rich, interactive settings show significantly less fossilization than those in passive, textbook-driven classrooms.

Address overgeneralization directly 🔗

Because overgeneralization is systematic, it responds well to explicit instruction. When a learner consistently writes *goed or *informations, the teacher knows exactly what hypothesis the learner has formed – and can design targeted input to expose the exception. Teachers should respond to overgeneralization errors with metalinguistic clues, repetition, or clarification requests that give learners the opportunity to detect and correct the error themselves, rather than simply supplying the right answer.

Balance fluency and accuracy 🔗

A persistent finding in SLA research is that classrooms which emphasize communication without sufficient attention to form allow errors to fossilize more readily. Learners who focus exclusively on getting their message across, while neglecting grammatical accuracy, are more susceptible to stabilizing non-target forms. The implication for teachers is not to abandon fluency-focused activities, but to build in regular, focused attention to form – particularly in areas where L1 interference or overgeneralization is known to be high for that learner group.

Recognize that each learner’s interlanguage is unique 🔗

Interlanguage theory emphasizes the individuality of each learner’s experience. Two students at the same proficiency level, but with different L1 backgrounds, will produce different errors, face different obstacles, and respond to different interventions. This calls for differentiated feedback and instruction that accounts for each learner’s linguistic background, not a one-size-fits-all approach to grammar correction.

From theory to classroom reality 🔗

Selinker’s interlanguage hypothesis built on Pit Corder’s earlier work on the significance of learner errors and became the foundation of modern second-language acquisition research. Its most lasting contribution to teaching is this: learners are not deficient native speakers – they are active builders of a temporary grammatical system that, with the right conditions, will evolve toward the target language. The teacher’s job is to understand where in that continuum each student is, what is holding their interlanguage in place, and what kind of feedback or input will push it forward.

Interlanguage research also reminds us that the study of second language acquisition is as much about the psychology of learning as it is about linguistics. The errors students make are windows into their cognitive process – systematic, logical, and ultimately, teachable.

What do you think? If interlanguage errors are a natural and necessary part of learning, how should teachers decide which errors to correct immediately and which to let evolve on their own? And how might knowing a student’s L1 background change the way you plan your grammar lessons?

References

1. https://www.ebsco.com/research-starters/language-and-linguistics/interlanguage
2. https://en.wikipedia.org/wiki/Interlanguage
3. https://www.intechopen.com/chapters/83784
4. https://www.sciencedirect.com/topics/social-sciences/interlanguage
5. https://www.tesl-ej.org/ej22/a1.html
6. https://files.eric.ed.gov/fulltext/EJ1082599.pdf
7. https://rsisinternational.org/journals/ijriss/articles/fossilization-in-second-language-performance-the-concern-of-ultimate-attainment/
8. http://tesl-ej.org/ej20/a2.html
9. https://duermueller.tripod.com/interlanguage.html
10. https://www.researchgate.net/publication/42386331_Implication_of_IL_Fossilization_in_Second_Language_Acquisition
11. https://www.atlantis-press.com/article/125983213.pdf
12. https://en.wikipedia.org/wiki/Larry_Selinker