I googled "most common words in English" and opened the top few sites. I found these two quotes right off (different websites):
Quote:Based on the combined results of British English, American English and Australian English surveys of contemporary sources in English: newspapers, magazines, books, TV, radio and real life conversations - the language as it is written and spoken today.
Quote:This is a list of the 3000 most commonly used words in American newspapers and popular magazines. We compiled this list by taking forty newspapers and magazines and simply sorting the words and then counting the frequency of the words.
I suspect that's how most of these lists are made. As far as I know, there is no single "corpus" that's universally accepted. One reason would be the difference in English as it's used in different parts of the world. The first site quoted above counted words in publications from many countries and sources; the second quote is from a site that used a rather small sample of American publications. The "corpuses" (corpi?) are probably similar, especially at the most-used part of the list...
Izzat substantive enuf? Pay attention to Phoenix -- she speaks wisely.