Question

When we say what did he see ( as example) and when we say what he saw .

answer me please

Answer 1

Look, if you use a linking verb, which is in this example the word "did", you make a question 

?What did you do

?What do they eat

But if you delete the linking verb, so the word "what" is not a question tool here, it's a relative clause. You use this kind of them to describe something you casted in your sentence

I don't know what you do there

Sorry if you didn't understand me

Comments

ya ,I know this already but I am talking about questions
I listen people say what happened?  not what did happen?

how  do you explain this please ?

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I can add to that problem
:You can also say
?Who took my pen
?What makes you feel that
?What's seems to to be the problem

And my answer is "of the top of my head" : if there is a subject in the question there is a linking verb, unless, there is no linking verb
And I've casted a question like that earlier
Maybe mr. Ibrahim will answer you better than me