jäh
sudden
Sudden, abrupt, and unexpected, often with a frightening or negative connotation.
Die Feier fand ein jähes Ende, als der Strom ausfiel.
The celebration came to a sudden end when the power went out.

sudden / steep
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sudden
Sudden, abrupt, and unexpected, often with a frightening or negative connotation.
Die Feier fand ein jähes Ende, als der Strom ausfiel.
The celebration came to a sudden end when the power went out.

The word 'jäh' (sudden / steep) belongs more to written language or an elevated style. It is used less often in normal everyday conversation.
Sein jäher Zorn erschreckte alle Anwesenden.
His sudden anger frightened everyone present.
You can recognize the meaning from the noun: If 'jäh' refers to an event or feeling, it means 'plötzlich' (sudden), e.g. 'jähes Ende' (sudden end). If it refers to a landscape, it means 'steil' (steep), e.g. 'jäher Abhang' (steep slope).
Der Film nahm eine jähe Wendung.
The film took a sudden turn.
In the meaning 'plötzlich' (sudden), 'jäh' is often connected with negative events: 'ein jähes Ende' (a sudden end), 'ein jäher Schreck' (a sudden fright), or 'ein jäher Stopp' (a sudden stop).
Durch den lauten Knall wurde die Stille jäh unterbrochen.
The silence was suddenly interrupted by the loud bang.
The meaning 'steil' (steep) is used almost exclusively to describe landforms such as slopes, precipices, or rock faces.
Unter uns tat sich ein jäher Abgrund auf.
A steep precipice opened up below us.
suddenly
'plötzlich' and 'jäh' both describe something that happens unexpectedly and quickly. 'plötzlich' is neutral and fits in many everyday situations. 'jäh' sounds stronger and often more serious. It is often used when something unpleasant, frightening, or harsh comes to an end, for example a dream, a hope, or a movement. For that reason, 'jäh' is not always a simple replacement for 'plötzlich'. If you only mean the element of surprise, use 'plötzlich'. If the moment feels harsh or negative, 'jäh' can fit well.

Der laute Knall riss die Gäste jäh aus ihrem Gespräch.
The loud bang abruptly tore the guests out of their conversation.
Beim Frühstück hat plötzlich das Telefon geklingelt.
During breakfast, the phone suddenly rang.