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Question words keep their accent, even without question marks

Spanish question words like qué, dónde, cuándo and cómo always carry a written accent. Almost every one of them has an unaccented twin that looks identical but does a different job: que (that), donde (where, in a statement), cuando (when, in a statement), como (as, like). The accent is how you tell them apart on the page. It also reflects pronunciation, because the question word is stressed in speech and its twin is not.

English speakers tend to link the accent to question marks. They write ¿Dónde vives? correctly but then drop the accent in a sentence like No sé dónde vive, because it looks like an ordinary statement. In fact the accent stays, because the word is still asking something.

Word list
cuándoA1 when e.g. No sé cuándo vuelve dóndeA1 where e.g. No sé dónde vive por quéA1 why e.g. por qué quéA1 what e.g. No sé qué quiere cómoA2 how e.g. No sé cómo funciona cuálA2 which e.g. Dime cuál quieres cuántomA2 how much e.g. Pregúntale cuánto cuesta cuántosmA2 how many e.g. No sé cuántos vienen
How it works

The accent depends on what the word does, not on the punctuation around it. If the word asks for information (what? where? when? how? why? which? how much?), it takes the accent. This is true in a direct question with ¿...? and also in an indirect question, where the question is tucked inside a longer sentence after verbs like saber, preguntar, decir, explicar or no entender.

¿Dónde vives y cómo llegas al trabajo?
(Where do you live and how do you get to work?)
No sé dónde vive ni cuándo vuelve.
(I don't know where she lives or when she's coming back.)

A quick test: try rephrasing the sentence as a real question. No sé dónde vive hides the question ¿Dónde vive?, so the accent stays. Compare a sentence where donde simply points to a place that has already been mentioned. Nothing is being asked, so there is no accent:

La ciudad donde vivo es muy tranquila.
(The city where I live is very quiet.)
The words

qué and cuándo: qué asks what, and cuándo asks when. Their twins que (that, which) and cuando (when, as a connector) are very common, so it is easy to confuse them.

No sé qué hacer ni cuándo empezar.
(I don't know what to do or when to start.)
Mi hermano dice que está cansado.
(My brother says that he's tired.)

por qué: this is written as two words with an accent when it means why, including in indirect questions. The answer word porque (because) is one word with no accent.

Explícame por qué cuesta tanto.
(Explain to me why it costs so much.)
No voy a la fiesta porque estoy enferma.
(I'm not going to the party because I'm sick.)

cuál and cómo: cuál asks which one, and cómo asks how. Watch out for como without an accent. It means as or like, and it is also the verb form I eat.

¿Cuál prefieres, el rojo o el azul?
(Which one do you prefer, the red one or the blue one?)
No entiendo cómo funciona esta máquina.
(I don't understand how this machine works.)

cuánto and cuántos: cuánto asks how much and cuántos asks how many. Both keep the accent in every form (cuánta, cuántas) whenever they ask a question.

Pregúntale cuánto cuesta el billete.
(Ask him how much the ticket costs.)
No sé cuántos invitados vienen.
(I don't know how many guests are coming.)
Watch out
  • Exclamations count too. The same words keep the accent in exclamations, because they are still stressed and are doing their question-word job: ¡Qué frío!, ¡Cómo llueve!, ¡Cuánta gente!
  • The most common mistake is writing No sé que quiere. If you mean I don't know what he wants, you need No sé qué quiere. Without the accent, the sentence reads like a broken that-clause.
  • Four forms of why and because. Por qué means why. Porque means because. El porqué is a noun meaning the reason, as in No entiendo el porqué. Learners at A2 mostly need the first two.
  • Other question words follow the same rule, even though they are not in this list: quién, quiénes (who) and adónde (where to), as in No sé quién es.
Summary

If a word asks for information (what, where, when, how, why, which, how much, how many), give it an accent, whether or not there are question marks. Imagine the hidden question: if No sé dónde vive can become ¿Dónde vive?, the accent stays. If the word only links two parts of a statement, as in que, donde, cuando, como or porque, it has no accent.