just / precisely
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just then
Expresses that something happened just a moment ago or at exactly this moment. It is a colloquial alternative to 'gerade' (just/right now).
Ich wollte dich anrufen, aber du bist ja just in dem Moment zur Tür hereingekommen.
I wanted to call you, but you came through the door just then.
'Just' (just then/precisely) is a false friend of the English word 'just'. In German it means 'gerade eben' (just now) or 'ausgerechnet' (of all times/precisely), not 'nur' (only) or 'einfach' (simply).
Just als ich gehen wollte, hat das Telefon geklingelt.
Just as I was about to leave, the phone rang.
The word 'just' (just then/precisely) is perceived as somewhat more elevated or literary than the everyday 'gerade' (just/right now) or 'ausgerechnet' (of all times). It is often used to emphasize a moment in particular.
Er erreichte den Bahnhof und just in diesem Augenblick fuhr der Zug ab.
He reached the station, and just at that moment the train departed.
In the meaning 'ausgerechnet' (of all times/precisely), 'just' often expresses surprise, irony, or an unfortunate coincidence. It emphasizes that something happens at a particularly inconvenient or unexpected time.
Just an meinem freien Tag muss es den ganzen Tag regnen.
Precisely on my day off, it has to rain all day.
now
'jetzt' and 'just' can both point to a very near point in time. 'jetzt' is the normal word for the present moment and fits almost any situation. 'just' is more colloquial and often means 'just now' or 'at exactly this moment'. It can also emphasize that something happens at this particular time of all times. Learners may confuse the words because both are connected to 'gerade' (just / right now). As a practical rule: for neutral German, you usually use 'jetzt'. 'just' sounds more special and emphasizes the exact or surprising moment.
Just als die Verkäuferin die Tür schließen wollte, kam noch ein Kunde herein.
Just as the saleswoman was about to close the door, another customer came in.
Ich muss jetzt aussteigen, weil ich gleich bei der Arbeit sein soll.
I have to get off now because I'm supposed to be at work soon.