die Wendung
turn
A sudden and often unexpected change in a situation, story, or development.
Die Ermittlungen nahmen eine überraschende Wendung, als ein neuer Zeuge auftauchte.
The investigation took a surprising turn when a new witness appeared.

turn / expression
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turn
A sudden and often unexpected change in a situation, story, or development.
Die Ermittlungen nahmen eine überraschende Wendung, als ein neuer Zeuge auftauchte.
The investigation took a surprising turn when a new witness appeared.

'Die Wendung' (turn/expression) is derived from the verb 'wenden' (to turn). The basic idea of a turn or change appears in all meanings: a turn of events, a linguistic 'turn,' or a physical turn.
Er musste das Auto in der engen Straße wenden.
He had to turn the car around in the narrow street.
The context makes the meaning clear: if it is about a story or development, it means a change. If it is about language, it means an idiom or expression. If it is about sports or movement, it means a physical turn.
Die Geschichte nahm eine unerwartete Wendung.
The story took an unexpected turn.
In the meaning of 'Veränderung' (change), the fixed combination 'eine Wendung nehmen' (to take a turn) is almost always used. It is often expanded with adjectives like 'überraschend' (surprising), 'dramatisch' (dramatic), or 'positiv' (positive).
Durch die neue Information nahm der Fall eine entscheidende Wendung.
Because of the new information, the case took a decisive turn.
When 'Wendung' (expression) means an idiom, it is often combined with adjectives like 'fest' (fixed), 'bildlich' (figurative), or 'gebräuchlich' (common). You 'benutzt' (use) or 'verwendet' (use) an expression.
In seiner Rede benutzte der Politiker viele bildliche Wendungen.
In his speech, the politician used many figurative expressions.
development
'die Entwicklung' and 'die Wendung' can both describe a change in a situation. 'die Entwicklung' more often means the course of events over a certain period of time or the current state after several steps. 'die Wendung' is more sudden and often unexpected: a situation suddenly takes a different direction. Learners may confuse the words because both are used when talking about news or stories. If something happens slowly or becomes clear step by step, 'Entwicklung' fits. If something takes a surprising turn, 'Wendung' is more precise.

Der Streit nimmt eine gute Wendung, als sich beide entschuldigen.
The argument takes a positive turn when both of them apologize.
Im Zug liest meine Schwester einen Bericht über die Entwicklung der Preise.
On the train, my sister reads a report about the development of prices.