I am not a native speaker of English. I am in the learning phase. 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 verbs. 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)
I am unable to get my head around it. If anyone can clear this confusion, I will be highly obliged. Thank you.