既然 + Established Fact, + 就/那 + Conclusion
既然 (jìrán) means "since" or "now that" and introduces a fact or situation that has already occurred or been established. The second clause then draws a logical conclusion or suggestion based on that established fact.
既然你已经决定了,那就去做吧。
Jìrán nǐ yǐjīng juédìng le, nà jiù qù zuò ba.
Since you've already decided, then go ahead and do it.
既然来了,就多待一会儿吧。
Jìrán lái le, jiù duō dāi yíhuìr ba.
Since you're already here, stay a while longer.
既然大家都同意,我们就按这个计划走。
Jìrán dàjiā dōu tóngyì, wǒmen jiù àn zhège jìhuà zǒu.
Since everyone agrees, let's go with this plan.