Expressing "since" with "jiran"

既然 (jìrán) means "since", and is often followed by 就 to mean "since...then...." It is used like this:

既然 followed by an affirmative sentence

This construction is not generally used to express a result, but more of a suggestion or inference deduced from the first fact.

既然＋Fact, 那 / 就 / 那就＋Suggestion / Inference

你既然不想说，那就别说了。Since you don't want to speak, just don't say it. a suggestion)
既然你那么喜欢，那就买吧。 Since you like it so much, buy it then. a suggestion)
她既然刚刚分手，那现在应该是单身吧。 Since she just had a breakup, she should be single now. an inference)
既然你们是好朋友，那他肯定会帮你吧。Since you guys are good friends, he will definitely help you. an inference)
既然大家都不感兴趣，就换个话题吧。Since nobody is interested, let's change the topic. a suggestion)

The suggestion or inference deduced from the first fact can come in the form of rhetorical question. In this case, “那” can be omitted.

既然 + Fact, （那 +）rhetorical question

你既然不喜欢他，为什么还想跟他出去？Since you don't like him, why do you want to go out with him? a suggestion)
既然你已经决定了，为什么现在后悔？Since you've made your decision, don't regret it then. a suggestion)
既然他那么忙，你为什么还去打扰他？Since he's so busy, why would you bother him? a suggestion)
既然已经分手了，为什么还要见面？Since we broke up, why would we still see each other? a suggestion)
既然你男朋友那么有钱，那他为什么不能大方一点？Since your boyfriend is so rich, why can't he be more generous? a suggestion)