I am not a native speaker of English. Yesterday my teacher was teaching about Future Perfect Tense, and he said that there are some helping verbs which cannot be preceded by another helping verb. Such as has, will, shall, should, ought to, must etc. And he gave an example of following sentence.
He will has written the essay. (Right)
He will have written the essay. (Wrong)
Please explain why this is so, I am having trouble understanding.