Why que and not quoi?

Question

I am confused about the differences between que and quoi and why you could not use quoi instead of que in some of these.

Accepted Answer

The lesson links to a more detailed discussion of other question forms. 
You cannot directly replace ‘quoi’ with ‘que’ in the same position in a sentence (question), as they are used in different question structures. Que is used at the start of formal inverted questions, quoi is used at the end of ‘statement questions’. 
If just asking the truncated question ‘what?’, you use ‘quoi?’. (Common informally, but not for formal situations, just as with ‘what’ in English).
https://progress.lawlessfrench.com/revision/grammar/how-to-ask-questions-with-qui-que-quoi-quand-ou-comment-pourquoi-and-combien