Language Transfer refers to speakers or writers applying knowledge from one language to another language.
Positive and Negative Transfer
When the relevant unit or structure of both languages is the same, linguistic interference can result in correct language production called positive transfer.
For example, Spanish speakers learning English may say “Is raining” rather than “It is raining”, leaving out the subject of the sentence. French speakers learning English, however, do not usually make the same mistake. This is because sentence subjects can be left out in Spanish, but not in French.
Language interference is most often discussed as a source of errors known as negative transfer, which occurs when speakers and writers transfer items and structures that are not the same in both languages.
Conscious or Unconscious Transfer
Transfer may be conscious or unconscious. Consciously, learners or unskilled translators may sometimes guess when producing speech or text in a second language because they have not learned or have forgotten its proper usage. Unconsciously, they may not realize that the structures and internal rules of the languages in question are different. Such users could also be aware of both the structures and internal rules, yet be insufficiently skilled to put them into practice, and consequently often fall back on their first language.
More on Second Language Acquisition
If you want to learn some more about other second language acquisition topics, make sure that you take a look at this page Second Language Acquisition