This is a common question for those who are learning Malay.

It is used in WH questions; What (Apa), Why (Kenapa), Where (Di mana), When (Bila),How (Bagaimana).

Here some important notes. When you add "kah", it is formal and mainly used in essays.

For example: 

1) Apakah itu? (What is that?)

2) Kenapakah awak ambil jurusan itu? (Why do you take that course?)

3) Di manakah kereta awak sekarang? (Where is your car now?)

4) Bilakah kita pergi ke sana? (When we will go there?)

5) Bagaimanakah mesin ini berfungsi? (How does this machine work?)

If you ask these questions without "kah", then it will be more informal, natural and applicable in daily conversation. Note that when you remove "kah", some of WH questions are put at the back of the questions. 

For example:

1) Itu apa? = grammartically correct, informal, used in daily conversation

    Apa itu? = grammartically incorrect, informal, used in daily conversation  

2) Kenapa awak ambil jurusan itu? = informal, widely used.

    Awak ambil jurusan itu kenapa?= Used in particular context. It is used when you want to put stress on the question. In this question, you really want to know the reason why he/she takes this course.

3) Kereta awak sekarang di mana? = grammartically correct, informal, used in daily conversation.

    Di mana kereta awak sekarang? = grammartically incorrect, informal, used in daily conversation.

    Kat mana kereta awak sekarang? = Kat mana is slang word for di mana, widely used in daily conversation, grammartically incorrect.

4) Kita pergi ke sana bila? = grammartically correct, informal, used in daily

    Bila kita pergi ke sana?= grammartically incorrect, informal, used in daily

5) Macam mana mesin ini berfungsi?= "bagaimana" is replaced by "macam mana", widely used in daily conversation

    Bagaimana mesin in berfungsi?= sounds odd if used in daily conversation