A W-question is used to ask about something that is unknown. This question cannot be answered with "yes" or "no". W-questions start with a question word, usually beginning with W, e.g.: Woher …? Wo …? Wie …?
Warum haben Sie uns diese langsamen Tablets verkauft?
Wie bekomme ich mein Geld zurück?
Revision: indirect interrogatives with a question word
A W-question can also be asked indirectly and a subordinate clause formed. This subordinate clause is usually the object of the main clause. In this case, the main clause cannot generally stand alone. The main clause can be a declarative sentence or an interrogative. If the main clause is declarative, there will be a full stop at the end. If the main clause is an interrogative, there will be a question mark at the end.
Declarative sentence + indirect interrogative:
Ich möchte wissen, warum Sie uns diese langsamen Tablets verkauft haben.
Interrogative + indirect interrogative:
Können Sie mir sagen, wie ich mein Geld zurückbekomme?
Indirect interrogatives are often used to formulate questions especially politely. However sometimes the question is ironic. In this case, no answer is expected to what is merely a complaint:
Wissen Sie, wie oft ich heute schon angerufen habe?
Grammatical terms in German:die W-Frage: W-questions are open questions that cannot be answered with "yes" or "no". They are also called "Ergänzungsfragen". W-questions start with a question word usually beginning with the letter W.