既然 means "as" , you can put the "as" to the beginning of the sentence. the meaning is so nearly to "因为", because the ahead part deduce the tail part. And in this case, "既然" contains a little of meaning of "the reason that" , or "so ...(conj.)". In Chinese, behind the "既然", usually need to be add the“就”. it's the habit using. The another one "居然", it's so simply, it have a meaning of "surprised". in fact, in some case of you speak in Chinese, it can be not clearly to expect a meaning like "unexpected" in Chinese, just a "unexpected" feeling. A sample, Li says to Wang: "Oh my god!! you got the PSP!!" it can be tranlated: 小李对小王说: "哦天啊！你居然有了PSP！" In this case, you can feel unexpected, but do not translate to be "意外地". because Li is so unexpected, not Wang is. The "居然" is who Li be feel supprised. this is the little diiferent.

既然 means：since; as; now that ； and usually it is a precondition in the case 居然，like you said it is unexpected， and usually you feel surprised. example: 既然你知道错误了，为什么不及时纠正呢？ Since you knew you'd made a mistake, why didn't you put it right immediately? 既然他执意要去，你就答应吧。 Seeing as he's determined to go, why don't you just agree to it? 教师节到来的那一天，李欢欢居然没有给老师送礼物。 聪聪考了双百分，明明当上了班长，居然都不请我们吃饭。

you should use juran when you are suprised with the resuld that you didnt know in advanc. For example,我真想不到，偷东西的人居然是你。 For jiran,i dont know how to explain it,but i will give u an example. 既然你已经答应了他，你就应该履行你的承诺。