What is Interlanguage?
　　The learners of a second language (L2) start out with their own language, which we call source language . They are on their way to learn a target language (TL). All that lies inbetween we call interlanguage. All L2 speakers are on some stage of interlanguage. Beginners are closer to their source language (SL), experts of L2 are closer to the target language. And if we don't continue with our studies, our interlanguage competence may even decrease. People who have lived in foreign countries for a long time are often so close to the target language that they hardly differ from native speakers. There are some features of interlanguage which are worthwhile to look at. They play an important role in the learning process. Everybody experiences their effects in language learning.
●Fossilization.
　　At a certain stage the learner ceases to learn new aspects of the TL. Although perhaps capable to express herself in a grammatically correct way, the learner here does not proceed to explore the great reservoir of language any further in order to express herself in a more refined and sophisticated manner.
●Regression.
　　The learner fails to express herself in areas (phraseology, style or vocabulary) that he or she had mastered at an earlier point in time.
●Overgeneralization.
　　The learner searches for a logical grammar of the TL that would cover every aspect of the language, or seeks to find every aspect of existing grammars confirmed in the living language. In doing so, the learner draws on aspects of the target language already earned and overuses them.
Overelaboration.
●prefabrication
　　The learner wants to apply complex theoretical structures to contexts that may call for simpler expression.
●Interference from L1 (or L3) ,
　　with phonological interference being the most common example. Syntactic interference and semantic interference are also possible, e.g., so-called false friends. These are words that exist in the source language as well as in the target language. However, their meaning or use might differ substantially, as in the German "Figur" vs. the French "figure" (="face"), or the English "eventually" vs. the German "eventuell" (="possibly").
●Variable input.
　　This refers to the quality of education in the TL, the variety and extent of exposure to the TL and the communicative value of it to the learner. This is why the design of learning material and contact with many TL native speakers plays a vital role in learning a new language.