Although porque, por qué, por que, and porqué have related meanings, they are not interchangeable. If you find them confusing as a Spanish student, you're in good company: Native speakers often write them incorrectly as well.
Por qué typically is used in questions, meaning "why":
- ¿Por qué celebramos el 16 de septiembre? (Why do we celebrate September 16?)
- ¿Por qué estamos aquí? (Why are we here?)
- ¿Por qué no citas tus fuentes? (Why don't you cite your sources?)
Por qué is also sometimes used in statements that form an indirect question. In such cases, it usually is still translated as "why."
- Dime por qué las noches son tan largas. (Tell me why the nights are so long.)
- Quiero saber por qué se usa el prefijo "www" en las páginas Web. (I want to know why the prefix "www" is used for Web pages.)
Porque typically means "because":
- Es simple porque se basa en el concepto de igualdad. (It is simple because it is based on the concept of equality.)
- Perdónalos, porque no saben lo que hacen. (Forgive them, because they don't know what they're doing.)
- Voy al restaurante porque tengo hambre. (I'm going to the restaurant because I'm hungry.)
- —¿Por qué sales? —Porque estoy aburrida. (Why are you leaving? Because I'm bored.)
Por qué and porque are far more common than the following two usages. If you're a beginner at Spanish you can probably safely stop here.
Por que occurs when que as a relative pronoun follows the preposition por. If that sounds confusing, think of por que as meaning "for which," although it is often translated as "that" or "why."
- Esa es la razón por que he querido salir. (That is the reason for which I wanted to leave.)
- El motivo por que llegué tarde fue el paro de buses. (The reason for which he arrived late was the bus stoppage.)
Por que also occurs when que follows a verb phrase using por. For example, the phrase for "to worry about" is preocuparse por. Here is an example where the phrase is followed by que: Se preocupa por que las soluciones sean incompatibles. (She is worried that the solutions will be incompatible.)
Finally, porqué is a masculine noun, meaning "reason," much like using "why" as a noun in English:
- No comprendo el porqué de la violencia doméstica. (I don't understand the reason for domestic violence.)
- Están acostumbrados a tomar decisiones, no a explicar los porqués. (They are used to making decisions, not at explaining the reasons.)