Learn Hardcore Chinese: jìrán shuǐlóngtóu lòushuǐ le, jiù mǎshàng guān zǒngfá. - Since the tap is leaking, close the main valve immediately.

Breakdown of jìrán shuǐlóngtóu lòushuǐ le, jiù mǎshàng guān zǒngfá.

Used after a verb. Marks that an action is completed.

What does 既然 mean here, and how is it different from 因为?

既然 means something like since, now that, or given that.

In this sentence, it introduces a fact that is already accepted:

Compared with 因为:

因为 simply gives a reason: because
既然 often means since this is already true / since we already know this, the next step should follow naturally

So 既然 often sounds more like:

That is why it often appears with 就 in the next clause.

Why is 就 used after 既然?

This is a very common pattern:

It means:

In this sentence:

The 就 marks the logical result, reaction, or next step.

It does not mean then in a time-sequence sense only; here it means more like:

Without 就, the sentence could still be understood, but 既然 ... 就 ... is the standard and very natural pairing.

Why is there a after ?