The list below of most common words in English cannot be definitive. It is based on an analysis of the Oxford English Corpus of over a billion words, and represents one study done by Oxford Online, associated with the Oxford English Dictionary. This source includes writings of all sorts from "literary novels and specialist journals to everyday newspapers and magazines and from Hansard to the language of chatrooms, emails, and weblogs", unlike some sources which use texts from only specific sources.
The Reading Teacher's Book of Lists claims that the first 25 words make up about one-third of all printed material in English, and that the first 100 make up about one-half of all written material.
Note that "word" may mean either a word form (essentially, a distinct spelling), or a lexeme (essentially, a "base" word or dictionary-entry). For instance the lexeme "be", listed below, includes occurrences of "are", "is", "were", "was", etc. Note also that these top 100 lemmas listed below account for 50% of all the words in the Oxford English Corpus.
Source:
Parts of speech
Source: