zusammenhängend
connected
Describes things that are directly connected to one another spatially or form an uninterrupted unit.
Die beiden Grundstücke bilden eine große, zusammenhängende Fläche.
The two plots of land form one large, connected area.

connected / coherent
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connected
Describes things that are directly connected to one another spatially or form an uninterrupted unit.
Die beiden Grundstücke bilden eine große, zusammenhängende Fläche.
The two plots of land form one large, connected area.

'Zusammenhängend' (connected/coherent) is the present participle of the separable verb 'zusammenhängen' (to be connected). It literally describes things that 'hang together' or are connected with one another, whether spatially or logically.
Er formulierte einen langen, zusammenhängenden Satz.
He formulated a long, coherent sentence.
The adjective 'zusammenhängend' (connected/coherent) is closely related to the noun 'der Zusammenhang' (context, connection). If something is zusammenhängend, it has a clear connection.
Damit die Geschichte zusammenhängend wirkt, muss der logische Zusammenhang stimmen.
For the story to seem coherent, the logical connection has to work.
You can tell whether a spatial or logical connection is meant from the noun. With concrete things like 'Fläche' (area) or 'Gebiet' (region), it is about space. With abstract things like 'Text' (text) or 'Geschichte' (story), it is about logic.
Die drei Zimmer bilden eine zusammenhängende Wohnung.
The three rooms form one connected apartment.
In the logical meaning, 'zusammenhängend' (coherent) is often used with verbs of speaking or writing. Typical phrases are 'zusammenhängend erzählen' (to tell coherently), 'zusammenhängend argumentieren' (to argue coherently), or 'zusammenhängend schreiben' (to write coherently).
Nach dem Schock konnte er den Vorfall nicht zusammenhängend schildern.
After the shock, he could not describe the incident coherently.
The phrase 'damit zusammenhängend' (connected with that) often comes at the beginning of a sentence or clause. The conjugated verb then follows directly in position 2.
Die Kosten sind gestiegen. Damit zusammenhängend müssen wir die Preise erhöhen.
The costs have risen. Connected with that, we have to raise prices.
closed
'geschlossen' and 'zusammenhängend' can both describe parts forming a unit. 'geschlossen' emphasizes that something is continuous, complete, or rounded in itself. 'zusammenhängend' emphasizes that the parts are connected to each other, either spatially or in terms of content. With an area or a series, both can fit, but 'zusammenhängend' makes the connection between the parts clearer. With a text or an explanation, 'zusammenhängend' means that the train of thought is easy to follow.

Anna konnte die Geschichte nach dem Unfall wieder zusammenhängend erzählen.
After the accident, Anna was able to tell the story coherently again.
Der Zaun bildet eine geschlossene Linie um den Garten.
The fence forms a continuous line around the garden.