快速答案
四种写法分别用于提问、回答、句末提问和名词。
可靠的方法是先判断这个词在句子里的功能，再翻译。
每个词的作用
por que
why or for which
Use por que for a direct or indirect question, or por plus the relative que.
porque
because
Use porque for an answer, explanation, or cause.
por quê
why at the end
Use por quê for a question form placed at the end of a clause.
porquê
the reason
Use porquê for a noun, normally after an article or other determiner.
如何选择
- por que: Choose por que if the sentence points to a direct or indirect question, or por plus the relative que.
- porque: Choose porque if the sentence points to an answer, explanation, or cause.
- por quê: Choose por quê if the sentence points to a question form placed at the end of a clause.
- porquê: Choose porquê if the sentence points to a noun, normally after an article or other determiner.
例句
Por que você saiu cedo?
Why did you leave early?
Saí cedo porque estava cansado.
I left early because I was tired.
Você saiu cedo por quê?
Why did you leave early?
Ninguém explicou o porquê.
Nobody explained the reason.
常见误区
Do not choose by sound; they sound alike. Check whether the expression asks, explains, ends the clause, or names a reason.
放到语境中练习
从你的真实生活中选一个句子，写两遍：一次用 por que，一次用 porque。如果两个版本都可以，说明意思怎样变化。