Since ... then ... with 既然…就 — Chinese grammar (HSK 4)

既然 + Known fact，(Subject) + 就 + Conclusion

既然 (jìrán) means "since / now that" and introduces a fact both speakers already accept, not a new reason. The second clause, with 就 after its subject, draws a conclusion or makes a suggestion from that fact. Compare 因为, which gives new information; 既然 builds on something already established.

Examples

既然你不舒服，就回家休息吧。 Jìrán nǐ bù shūfu, jiù huí jiā xiūxi ba.	Since you are not feeling well, go home and rest.
既然决定了，就别后悔。 Jìrán juédìng le, jiù bié hòuhuǐ.	Now that you have decided, do not regret it.
既然大家都同意，我们就按照这个计划做。 Jìrán dàjiā dōu tóngyì, wǒmen jiù ànzhào zhè ge jìhuà zuò.	Since everyone agrees, we will go ahead with this plan.

Tip: Use 既然 only for facts already on the table; for a brand-new reason use 因为.