…or you cannot compete in the contest.
Seems easy enough, right? We have a verb, and an adjective. But enough of that boring English lesson stuff. When I have seen these two words interchanged, it is because the writer used the verb as an adjective, and the adjective as the verb.
Some ideas to help all of us out. You can be an alternate in the spelling bee. You can alternate turns of who washes the dishes.
If you do not want to wash the dishes at all, you need to find an alternate, or better yet, come up with an alternative plan. Such as eating out. Then no one is washing the dishes. Well; no one except the employees at the restaurant.
Alternative is a substitution; a different choice besides the one proposed.
Alternate means take turns, and alternative means a different choice.