i am often confused using "do" with interrogative words such as why,how,where and so on.?

Question

when do or does
is a must portion of the sentences. please help.

Answer 1

The use of "do " & "does" depends on the subject of the sentence. This applies whether you use an interrogative word or not. "Does" always relates to a third person. "Do" relates to first or second person.

When does he leave ~ when do you leave ~
Does he know? ~ Do you know?

Answer 2

When you're using *I, you, they, we* -use "do."
I do
you do
they do
we do
(And any plural subject: birds do)

When you're using *he, she, it* -use "does."
he does
she does
it does
(And any singular subject: bird does)

The question words, who, what ,where, etc. do not matter.

Answer 3

Why do you do that?
How does he do that?
Where are they going?