What this is
Five question words, one shared mechanism. Each of où (where), quand (when), comment (how), pourquoi (why), and combien (how much/many) slots into exactly the same frames you already have — est-ce que, inversion — the same way qui and que did. Once you’ve got the frame, adding a new question word to it is vocabulary, not new grammar.
The rule
Any of the five combines with est-ce que or inversion:
Où est-ce que tu habites ? Où habites-tu ? Where do you live?
Quand est-ce que tu manges ? Quand manges-tu ? When do you eat?
Comment est-ce que tu voyages ? Comment voyages-tu ? How do you travel?
Pourquoi cherches-tu ... ? Why are you looking ... ?combien almost always pairs with de + noun to ask about quantity:
Combien de frères as-tu ? How many brothers do you have?
Combien d'amis avez-vous ? How many friends do you have?This is the exact pattern from beaucoup de and trop de in partitive-articles — bare de, no article in between, regardless of the noun’s gender.
où est / où sont is common enough to treat as a fixed chunk:
Où est le restaurant ? Where's the restaurant?
Où sont les enfants ? Where are the children?pourquoi is naturally answered with parce que (because):
Pourquoi es-tu fatigué ? Parce que tu travailles beaucoup.
Why are you tired? Because you work a lot.Why it works this way
These five words are all adverbs that specify a circumstance of the action — place, time, manner, cause, or quantity — which is exactly the role adverbs play in an ordinary statement too (il travaille beaucoup, il travaille ici). Turning a statement into a question by fronting one of these words and applying est-ce que or inversion is the same operation you already learned for qui/que; the words themselves are just new content plugged into an existing slot.
Edge cases
combien de never takes an article
Like beaucoup de, trop de, and assez de before it, combien de goes straight to the noun — combien de patience, never combien de la patience. The quantity word already tells you “some unspecified amount”; an article on top would be redundant.
où est/sont as a fixed chunk
You could build où est le restaurant from où + est + le + restaurant piece by piece, but it’s frequent enough that it’s worth learning as a single retrievable chunk, the way you’d memorize “where’s the bathroom” in any language rather than re-deriving it.
What English speakers get wrong here
The most common slip is reaching for an article after combien de out of habit — the same mistake that shows up with beaucoup and trop, now with a new trigger word. The second is trying to answer pourquoi with parce (dropping que) by analogy with English “because” standing alone — French always needs the full parce que; que never drops here the way it can elsewhere.