An interlanguage is a type of speech or writing developed by people during the process of learning a new language, when the learner is starting to gain proficiency in the new, or “target,” language but has not mastered it. It is a distorted form of the target language that contains errors caused by inappropriately using aspects of the learner's native language while trying to speak the target language, incorrectly applying the target language's grammar or pronunciation rules or trying to express concepts in the target language by using more basic words the learner already knows. This is normal during the process of learning a new language. Every interlanguage is specific to the person speaking it and evolves as he or she continues to learn the target language.
Alhough they are both formed from elements of multiple languages, an interlanguage should not be confused with a pidgin or Creole language. Pidgin language is an improvised form of communication created by two or more people who do not share a language in common, while a Creole language is a language that originally arose from a mixture of different languages but has become a natural language in its own right, with children in the society where it is spoken growing up with it as their native language. An interlanguage, on the other hand, is always unique to a particular individual and is by definition never anyone's first language, as it is partially a product of a different language that the speaker already knows.
Interlanguages typically contain elements of the speaker's original language. For example, in English an adjective appears before the noun it modifies, while in French the adjective usually comes after the noun. Thus, an English speaker learning French who knows that the French words for “green” and “fish” are vert and poisson, respectively, might call a green fish “un vert poisson,” when un poisson vert is actually correct. The interlanguage of a French person learning English might contain the opposite error, causing him or her to say things like “a fish green.”
An interlanguage can also contain errors caused by knowing the target language's general rules but following them too rigidly. A non-native speaker might conjugate irregular verbs according to the rules of regular verbs, similarly to the way small children learning their first language often do. This can produce mistakes like saying “goed” instead of “went” or “you am” instead of “you are,” for example.
Language learners may also over-apply previous lessons about how the target language differs from their native language. For instance, while adjectives in French usually follow the noun, there are exceptions. Petit, French for “small,” is an example of this. Once an English speaker has learned how French adjectives generally work, he or she might overgeneralize the knowledge and incorrectly refer to a small fish as “un poisson petit” rather than the correct un petit poisson. The particular way a learner incorrectly applies the rules of the target language depend on when and how they were learned in the first place. An English speaker who had not yet learned that most French adjectives follow rather than precede nouns would be unlikely to make a mistake like “un poisson petit,” for instance.
A French speaker learning English would not make a similar error on that basis, because in this case the word order is the same in both languages and English never, with very rare exceptions such as certain legal terms, places adjectives after nouns. He or she might make similar errors for other reasons, however. For instance, the French speaker might remember proper English word order for adjectives by thinking of it as the opposite of French, a rule that will give the correct order most of the time but in this case produces the incorrect “a fish small.” Thus, an interlanguage can also be influenced by how the learner mentally organizes and keeps track of what he or she has learned about the target language, the mental aids or shortcuts he or she uses, and so on.
Finally, an interlanguage can contain attempts to express things the learner has not yet learned in the target language, using his or her limited existing knowledge of it. This can involve vocabulary. If the only French word for a bladed, hand-held object that an English speaker knows is glaive, for example, he or she might refer to a bread knife, called a couteau a pain, as “un petit glaive,” which would actually mean “small sword” or “small spear.” It can also involve attempting to precisely and literally replicate sentences from the native language in the target language without understanding how things are normally phrased in the target language or the differing connotations of words in the target language.