The list below of most common words in English, like any superlative list, 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.
Note that the items listed may represent more than one actual word; they are lemmas. For instance the entry "be" contains within it the occurrences of "is", "was", "be" and "are". Note also that these top 100 lemmas listed below account for 50% of all the words in the Oxford English Corpus.