
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.

steep
(elevated style) Very steep or ruggedly descending. Usually used for terrain, rocks, or slopes.
Der Wanderweg führte an einem jähen Abgrund entlang.
The hiking trail ran along a steep precipice.
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 unexpected that happens quickly. Plötzlich is neutral, while jäh sounds more elevated and often emphasizes an especially abrupt, frightening, or negative change. Use plötzlich for a neutral statement and jäh for a sharp break.

Das freundliche Gespräch endete jäh, als die schwere Nachricht eintraf.
The friendly conversation came to an abrupt end when the serious news arrived.
Das Gespräch endete plötzlich, weil der Zug in einen Tunnel fuhr.
The conversation ended suddenly because the train entered a tunnel.