A view of language as a rule-governed system with hierarchical subsystems (phonology, morphology, syntax); focused more on form than meaning.

A theory of learning as habit formation through stimulus, response, and reinforcement; errors are seen as bad habits to be avoided.

A form in learner language that differs from target language norms; traditionally seen as a problem but later viewed as evidence of learning.

Mistakes that arise from the internal complexity of the L2 itself (e.g., confusing much and many in English).

Mistakes caused by transfer from the learner’s first language (e.g., saying “She has 15 years” under influence of Spanish “Ella tiene 15 años”).

The influence of one language on another when bilinguals or learners use both; often L1 affecting L2.

Errors that occur in all learners regardless of L1, reflecting universal stages of language acquisition (e.g., “I goed” for “I went”).

A learner strategy of not using difficult structures to escape possible errors (e.g., avoiding relative clauses).

An approach (Corder, 1967) that studies errors as evidence of the learner’s developing system (transitional competence).

The evolving and temporary linguistic system that learners create while moving toward the target language.

A mistake that is grammatically or formally incorrect and easily noticeable (e.g., “He don’t like it”).

A grammatically correct form that expresses the wrong meaning (e.g., “I want to know the English” instead of “I want to learn English”).

A learner’s independent linguistic system, influenced by both L1 and L2, systematic but dynamic, and subject to change.

A stage in interlanguage where progress stops, and errors become permanent despite further exposure or instruction.

The process where features from L1 influence the learner’s use of L2 (can be positive or negative).

Errors caused by the way language is taught rather than L1 (e.g., overusing certain forms drilled in class).

Simplifications learners use to reduce cognitive load (e.g., using only simple present tense).

Techniques learners use to get their message across despite gaps in knowledge (e.g., paraphrasing, word coinage, code-switching).

Applying an L2 rule too broadly (e.g., “choosed” instead of “chose”).