Interrogative adverbs also form open questions, only that they do not ask for a person or thing, but for certain circumstances like date, time or others, which is described by adverbial clauses. Also with interrogative adverbs the question can be form in two ways, with and without est-ce que.
Overview
quand = when
Quand est-ce qu' il est arrivé?
Quand est-il arrivé?
When did he arrive?
comment = how
Comment est-ce qu' il s' est rendu compte?
Comment s' est-il rendu compte?
How did he notice?
où = where
Où est-ce qu'il veut aller?
Où veut-il aller ?
Where does he go?
pourquoi = why
Pourquoi est-ce qu' il n' a pas répondu?
Pourquoi n' a-t-il pas répondu?
Why did he not answer?
combien* = how many / how much
Combien
de pays est-ce que tu as visités?
Combien
de pays as-tu visités?
How many countries did you visit?
* Interrogative adverbs that ask for a quantity use the partitive article de
(and not de + articles = du, de la, des).
The interrogative adverb
où can be used with prepositions (de = d' où, jusque = jusqu' où, par = par où) . One thing to remember is that where (où) and where to (où) is the same in French.