既然…就【jìrán…jiù】

既然 opens a clause stating something already known or accepted, and the second clause says what follows from it – usually a suggestion, a decision or a question: "since", "now that", "given that". The second clause usually carries 就, after its subject if it has one.

The second clause can also be a challenge: if you accept the first part, why don't you act on it?

那 or 那就 at the head of the second clause is a common spoken alternative to 就.

既然 or 因为

因为 gives a reason the listener may not know, and pairs with 所以 to report a cause and its effect. 既然 starts from a fact both sides already accept and uses it to argue for what should happen next. It doesn't take 所以, and it doesn't report a plain cause.

• × 既然下雨了，所以路很滑 → 因为下雨了，所以路很滑
• × 既然你不舒服，所以早点儿回家吧 → 既然你不舒服，就早点儿回家吧