Aside from the Killing Curse, I doubt there is a more famous spell in the HP-Verse than Harry's signature move, Expelliarmus. Besides, you can always look them up in the books, on Pottermore, or even online in places like the Harry Potter Lexicon.
So why are you spelling it "Expelleramus"? Or use "Incarceratous" instead of Incarcerous, to give just two examples?
Granted, JKR's spells were bastardized Latin at best, liberally mixed with English words that in themselves are rooted in terms which entered the language after 1066 through Norman French; she presumably did that so the meaning of her made-up spells was fairly obvious with minimal effort. But at least she used the correct spelling!
Why the HELL can't you?!?