What is a Negative Transfer?

A negative transfer is a process that happens during second language acquisition, where a learner of a foreign language applies rules that do not belong to the language he is speaking or writing. An example might be applying rules of the Spanish grammar while speaking English. In Spanish the subject does not have to be always expressed in an utterance, so one might improperly say "Is raining", while the English correct form would be "It is raining" with a pronoun indicating the subject that in Spanish can be omitted.