I briefly touched the matter of omdat and want earlier, in the post about conjunctions, but it turned out to be a bit more complex than  I thought. I found the explanation which made it quite clear to me on the forum of dutchgrammar.com. Just in case, I’ve decided to copy part of the conversation here, but please visit the forum of dutchgrammar.com for the original posts and  more.

Omdat versus Want

by Bieneke » May 26th, 2008, 12:04 am 

The semantical difference between ‘omdat’ and ‘want’ is not easy to explain. They both give a reason for something else in the sentence and they are often interchangeable. Yet, there is a subtle difference. I will try to explain it here. Perhaps, other people can add more examples to clarify it further.

Omdat

After ‘omdat’ (in the subordinating clause), you give the reason for ‘A’ (an action or state of affairs) in the main clause.

(A) Hij leert Spaans, (B) omdat hij volgend jaar naar Venezuela verhuist.
(A) He learns Spanish (B) because he is moving to Venezuela next year.

(A) Haar huis is blauw, (B) omdat blauw haar lievelingskleur is.
(A) Her house is blue (B) because blue is her favourite colour.

Want

We use ‘want’ (in a co-ordinating clause) to explain why you said ‘A’ in the main clause.

(A) We moeten snel gaan pauzeren, (B) want ik begin echt moe te worden.
(A) We must take a break soon (B) because I am really getting tired.

(A) Het zal wel gaan regenen, (B) want de lucht is erg donker.
(A) It is probably going to rain (B) because the sky looks very dark.

We cannot say: “Het zal wel gaan regenen, omdat de lucht erg donker is”, because ‘omdat’ implies a reason for the state of affairs. The dark sky will not cause the rain. The speaker simply explains why he said it will probably rain soon.

In most cases, we can use both ‘want’ and ‘omdat’.

(A) Ik doe de verwarming aan, (B) omdat het koud is.
(A) Ik doe de verwarming aan, (B) want het is koud.
(A) I switch on the heater (B) because it is cold.

In the first example, you inform us that you (decide to) switch on the heater as a result of the fact that it is cold. In the second example, you explain why you said that you switch on the heater.

I must admit the difference between the two is all but clear.

Bieneke

Ally wrote:

– Ik ben laat omdat mijn auto kapot is.
– I am late because my car is broken.

– Ik denk dat ik ziek ben want ik heb koorts.
– I think I’m sick because I have a fever.