Do/Does

[Vivienne]

How can I explain why this is incorrect:

"What colour eyes does he/she has?"

and

"What does your father is do?"

Thank you

[matter]

[Question word/phrase] + AUXILIARY + SUBJECT + MAIN VERB (bare infinitive) + ?

In 1 "has" is not the bare infinitive. Should be:
"What colour eyes does he/she have?"

In 2 "is do" is two verbs. Should be:
"What does your father do?"

See Question Word Questions

[jirkux]

The auxiliary (do, does, did) is kind of an "operator". It doesn't have any meaning it just forms the question and it likes to take on all the "grammar". It takes the third person -s so you don't need it any more:

He likes music.

What does he like?
(we don't need the -s any more with the verb like here, because it's included in the operator already).