All languages change and evolve. The English language has been evolving ever since the first settlers in the 1700s. Reasons for the change include the need for new words (all new things that are invented need a name), the different dialects and slang words of different age groups, education levels, and places in the country (a professor on the East coast probably does not talk the same way as a teenager on the West coast, and neither one of them would talk the same as a grandma from the south). The immigration of new words from other languages and the misinterpretation of the meanings or pronunciation of current words ("pease" once referred to a single or a group of the vegetable we now know as a "pea". People assumed "pease" was the plural, and the word pea was born).
The Early Changes
In 1735 the English were calling the American Language "barbarous," and referred to the "Americanisms" as barbarisms. For 100 years after the Revolutionary War, they continued to make fun of the language. The Americans were proud of their new language because it showed their new independence. In 1783, Noah Webster wrote "A Grammatical Institute of the English Language" which was used as a text book and was often referred to as the "Blue-backed Speller" because of its blue cover.