1- Language splitting:A language may split into two or more languages if the speakers become separated into two or more groups with little or no contact.
2-Language contact:Language contact also plays a major role in language change. If two groups of people speaking two different (possibly completely unrelated) languages come in close contact with each other (for trade, etc.), then each group’s language may begin to adopt features of the other’s. Vocabulary is frequently added to a language through language contact. (e.g.: skunk, moccasin, chipmunk from Native American languages). Less frequently phonetic, phonemic, and even syntactic, morphological and semantic borrowing can occur.
3-Borrowings: Borrowings frequently obscure the relationship of languages by “covering the tracks” of historical development.
Linguistics 