既然……，为什么……
Reach for this when you want to push back on something the other person already accepts as true — 'Since X is the case, why Y?' It's an argumentative move: you take a premise the listener has agreed to and then challenge a contradiction or strange behavior that follows from it. Common in debates, persuasive writing, op-eds, and the kind of pointed question a parent or friend asks when they think you're being inconsistent.
Structure
既然 [ACCEPTED PREMISE]，为什么 [CHALLENGE / CONTRADICTION]
jìrán..., wèishénme...
How to Think About It
既然 means 'given that' — it locks the premise as already established, not up for debate. The 为什么 then leans on that locked premise to expose a contradiction. It's a rhetorical pincer: you can't escape the question without either denying the premise or admitting the contradiction. Plain 'why' doesn't do this work — it just asks for a reason.
Examples
既然你不喜欢这份工作，为什么不辞职？
Jìrán nǐ bù xǐhuān zhè fèn gōngzuò, wèishénme bù cízhí?
Since you don't like this job, why don't you quit?
既然知道答案，为什么不告诉我？
Jìrán zhīdào dá'àn, wèishénme bù gàosù wǒ?
Since you know the answer, why aren't you telling me?
既然这么累，为什么还要熬夜？
Jìrán zhème lèi, wèishénme hái yào áoyè?
Since you're this tired, why are you still staying up late?
Common Mistake
Learners use 既然 when they mean a neutral 'because' (因为). 既然 specifically marks a premise the OTHER side has accepted; using it for your own private reason sounds strange — like you're attributing a belief to someone who never stated it.
既然我饿了，为什么不吃饭？
因为我饿了，所以我要吃饭。
Don't Confuse With
既然……就……
Same 既然 setup but ends in a suggestion ('then just...') instead of a question. '既然下雨了，就别去了' (Since it's raining, just don't go). Use 既然……，为什么…… when you want to challenge, not advise.
因为……所以……
Neutral cause-and-effect. Doesn't presume the listener already accepted anything. Pick 因为 for explanations, 既然 for arguments.
如果……，为什么……
Hypothetical — 'IF X were true, why...'. 既然 treats the premise as a fact already on the table; 如果 leaves it as a maybe.
Practice
____你已经决定了，为什么还在犹豫？
Show answer
既然
既然你不饿，____不吃饭？
Show answer
为什么
Put in order: [既然 / 为什么 / 喜欢她 / 你 / 不告诉她 / ，]
Show answer
既然你喜欢她，为什么不告诉她？
Translate to Chinese: 'Since you don't have time, why did you agree?'
Show answer
既然你没有时间，为什么答应了？
Write a sentence challenging a friend's inconsistent behavior, using 既然……，为什么……
Show answer
Example answer: 既然你想减肥，为什么还吃这么多甜食？ (Since you want to lose weight, why are you still eating so many sweets?)
Hear It in Real Episodes
This pattern appears in 1 Fluentide episode: