Since… then… / since… why bother (既然…那么/既…何必)

Structure 既然 + premise，那(么) + conclusion （rhetorical variant: 既 + premise，何必 + …）

既然 (jìrán) takes an already-accepted fact and draws a conclusion from it: «since (it's already so)…». Compared with 既然…就… (HSK 4, id hsk4-jiran-jiu), here the closing is 那(么) (more discursive and emphatic) or the formal rhetorical variant 既…何必/何须… The 既…何必… form contracts to 既 (no 然) and closes with a rhetorical question that negates: «if it's already decided, why keep hesitating?» — meaning there's no need. This variant packs reasoning and mild reproach into one clause, typical of essays and educated speech.

Examples

既jì然rán大dà家jiā都dōu同tóng意yì，那nà么me就jiù这zhè么me定dìng了le。
Since everyone agrees, then it's settled this way.
既jì已yǐ决jué定dìng，何hé必bì再zài犹yóu豫yù。
Since it's already decided, why hesitate any further?
既jì然rán事shì已yǐ至zhì此cǐ，那nà就jiù坦tǎn然rán面miàn对duì吧ba。
Since things have come to this, let's face them calmly.
既jì知zhī其qí不bù可kě为wéi，何hé须xū勉miǎn强qiǎng而ér为wéi之zhī。
If one already knows it cannot be done, why force oneself to do it?

Levels are indicative: they follow the standard Chinese Grammar Wiki (AllSet Learning) progression, not an official HSK 3.0 mapping. We never assign levels by eye.

This point and the other 370, with audio on every example, inside Wanyu.