The most commonly used words in written English

(Source: American Heritage Word Frequency Book)

1. The
2. Of
3. And
4. A
5. To
6. In
7. Is
8. You
9. That
10. It
11. He
12. For

These might be the most common words, but they're also the ones that will probably give you the most trouble, knowing them is one thing, knowing how to use them quite another.  Size can be deceptive. Tongue out