conjunction of wh- question

It's just difficult to understnd at first.

I assume you mean "You are going with Mary and who(m)?"

is the following question ungrammatical? if so, what's the reason?

Mary and who are you going with?
Incorrect. Consider that you are joining two separate questions..

Mary are you going with? Incorrect
Who are you going with? Correct

Anonymousis the following question ungrammatical?

Yes.

You can't say "I am going with Mary and who," unless you are Dr. Seuss writing a sequel to "Horton hears a Who"

A question has two beginning forms:
1) interrogative pronoun: Who are you going with?
2) the (helping) verb: Are you going with Mary?

Here is the way to ask yo

AlpheccaStarsYou can't say "I am going with Mary and who,"

Using a questioning intonation is an acceptable way of forming questions in English. It is far less ommonly used than some learners appear to believe, but there is nothing inherently incorrect about "You are going with Mary and who(m)?"