<P> Language transfer has been the subject of several studies, and many aspects of it remain unexplained . Various hypotheses have been proposed to explain language transfer, but there is no single widely accepted explanation of why it occurs . </P> <P> The primary factor affecting language acquisition appears to be the input that the learner receives . Stephen Krashen took a very strong position on the importance of input, asserting that comprehensible input is all that is necessary for second - language acquisition . Krashen pointed to studies showing that the length of time a person stays in a foreign country is closely linked with his level of language acquisition . Further evidence for input comes from studies on reading: large amounts of free voluntary reading have a significant positive effect on learners' vocabulary, grammar, and writing . Input is also the mechanism by which people learn languages according to the universal grammar model . </P> <P> The type of input may also be important . One tenet of Krashen's theory is that input should not be grammatically sequenced . He claims that such sequencing, as found in language classrooms where lessons involve practicing a "structure of the day", is not necessary, and may even be harmful . </P> <P> While input is of vital importance, Krashen's assertion that only input matters in second - language acquisition has been contradicted by more recent research . For example, students enrolled in French - language immersion programs in Canada still produced non-native - like grammar when they spoke, even though they had years of meaning - focused lessons and their listening skills were statistically native - level . Output appears to play an important role, and among other things, can help provide learners with feedback, make them concentrate on the form of what they are saying, and help them to automatize their language knowledge . These processes have been codified in the theory of comprehensible output . </P>

Is the output that a system is capable of achieving over a period of time