English has changed greatly, and English speaker of year 1300 wouldn't have understood the English of 500 years nor the English of today. Changes of every sort have taken place, along with sounds (phonetics), in their distribution (phonemics) and in grammar (morphology and syntax).
Large-scale shifts often occur in response to social, economic and political pressures. The needs of speakers drive language change. New technologies, industries, products and experiences simply require new words. For example, 'plastic', 'cell phones' and 'the internet' didn't exist in Shakespeare time. By using new and emerging terms, we all drive language change. But the unique way that individuals speak also fuels language change.
Vocabulary can change quickly as new words are borrows from other languages, or as words get combines or shortened. Some words are even created by mistake, such as 'pea'. Up until 400 years ago, 'pease' referred to a single pea or many peas. At some point, people mistakenly assumed that the word 'pease' was the plural form of 'pea' and a new word was formed.
'To google' has become universally known and many countries are developing their own internet slang. 'Text speak' is a mixture of often vowel-free abbreviations and acronyms, and is questioned whether 'text speak' is becoming a new language. People fill texts with abbreviations, however it is believed that only 10% of the words in average are not written in full.
