Dear Writers of English,
Never say "definately." It's not a word.
Instead, please use "DEFINITELY," as in "I definitely never say definately." The root of definitely is "finite," meaning "having bounds or limits." Sadly, there is no such thing as a "finate." Knowing the root will not only help you spell the word, but your knowledge of the English language will impress your friends...which is inherently why we spell things correctly anyway.