Learn Hardcore Latin: Etsi auspex sero venit, deductio ante noctem incipit. - Although the observer of auspices arrives late, the procession begins before night.

Etsi introduces a concessive clause. In this sentence, it means although or even though. It signals that the first idea might seem like an obstacle, but the main statement still happens anyway.

So the structure is:

Etsi auspex sero venit = the concessive clause
deductio ante noctem incipit = the main clause

Why is venit indicative and not subjunctive after etsi?

Because etsi normally takes the indicative when the speaker is treating the statement as a real fact.

So Latin does not automatically use the subjunctive after a word meaning although. Here the sense is straightforward: the lateness is presented as something real, not hypothetical.

Is venit present or perfect?

Without macrons, venit can be ambiguous.

venit = comes or is coming
vēnit = came or has come

So if the meaning you were shown uses comes, then read it as present. If it uses came, then it is the perfect vēnit.

This is a very common issue in Latin texts printed without macrons.