überraschend
surprising
In a way that one did not expect; unexpected or unforeseen.
Der Sieg der Außenseitermannschaft kam für alle überraschend.
The underdog team's victory came as a surprise to everyone.

surprising
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surprising
In a way that one did not expect; unexpected or unforeseen.
Der Sieg der Außenseitermannschaft kam für alle überraschend.
The underdog team's victory came as a surprise to everyone.

'Überraschend' (surprising) is the present participle of the verb 'überraschen' (to surprise). Many adjectives in German are formed this way by adding '-d' to the infinitive of a verb.
Das war eine überraschende Wendung in der Geschichte.
That was a surprising twist in the story.
This word can stand before a noun as an adjective or function as an adverb that describes a verb or another adjective. As an adverb, it remains unchanged.
Wir erhielten eine überraschende Nachricht.
We received a surprising message.
As an adverb, 'überraschend' (surprisingly) is often used to intensify or modify another adjective. Typical combinations are 'überraschend gut' (surprisingly good), 'überraschend einfach' (surprisingly easy), or 'überraschend schnell' (surprisingly fast).
Die Prüfung war überraschend einfach.
The exam was surprisingly easy.
suddenly
'plötzlich' and 'überraschend' both describe something you did not expect. 'plötzlich' emphasizes that something happens quickly and without warning. It often stands directly with the verb: something happens 'plötzlich' (suddenly). 'überraschend' puts more emphasis on the fact that the result or situation is unexpected for someone. It can also be used for things that do not happen in a single moment, for example 'ein überraschendes Ergebnis' (a surprising result). Learners may confuse the words because both have to do with the unexpected. If the quick moment is important, 'plötzlich' fits better; if the surprise is the main focus, 'überraschend' fits better.

Das Gespräch mit dem Chef war überraschend freundlich.
The conversation with the boss was surprisingly friendly.
Beim Spaziergang beginnt es plötzlich stark zu regnen.
During the walk, it suddenly starts raining heavily.