since (point)
Zeitpunkt — seit wann etwas geschieht
Point in time — since when something has been happening
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Wir kennen uns seit der Schulzeit.— We've known each other since school.
When an action started in the past and IS STILL ONGOING, German uses Präsens: "Ich lerne seit zwei Jahren Deutsch" (I have been learning German for two years). Russian also uses the present tense here, but English uses Present Perfect Continuous — a frequent confusion.