The structure 既然…就…
"Since..., then..."
The structure 既然...就... 
is used to express an implication or conclusion deduced from a situation.
is used to express an implication or conclusion deduced from a situation.Scheme
既然
situation
就
conclusion
It is often used in a compound sentence with two subjects. The first subject can be placed before or after 既然, and the second subject can be placed before 就.
If a sentence only has one subject, it can be placed either in the first part of the sentence or in the second part of the sentence.
Examples
既然他没来，就应该有堵车吧
Since he didn't come, there must be a traffic jam
jìrán tā méi lái, jiù yīnggāi yǒu dǔchē ba
既然谁也不喜欢这部电影，我们就换成另外一部吧
Since no one likes this film, let's switch to another one
jìrán shéi yě bù xǐhuan zhè bù diànyǐng, wǒmen jiù huàn chéng lìngwài yí bù ba
You can also use 那 , 那就 and 所以 instead of 就.
, 那就 and 所以 instead of 就.Examples
你既然喜欢了，那就直接买吧
If you like it, just buy it
nǐ jìrán xǐhuan le, nà jiù zhíjiē mǎi ba
既然他是你的男朋友，那他一定会陪你去这个活动吧
Since he's your boyfriend, he'll go to this event with you anyway
jìrán tā shì nǐ de nánpéngyou, nà tā yídìng huì péi nǐ qù zhè ge huódòng ba