
zusammenhängen mit
to be related to
To have a connection or a causal, logical, or thematic relationship to something.
Die hohe Arbeitslosigkeit hängt mit der schwachen Konjunktur zusammen.
The high unemployment is related to the weak economy.
to be connected / to be related
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to be related to
To have a connection or a causal, logical, or thematic relationship to something.
Die hohe Arbeitslosigkeit hängt mit der schwachen Konjunktur zusammen.
The high unemployment is related to the weak economy.

to be connected
To be physically connected to each other or to form a unit.
Die beiden Grundstücke hängen direkt zusammen und bilden eine große Fläche.
The two plots of land are directly connected and form one large area.
'Zusammenhängen' (to be connected/related) is a separable verb. In a main clause, the prefix 'zusammen' stands at the end, while in a subordinate clause it remains attached to the verb.
Diese beiden Probleme hängen eng zusammen.
These two problems are closely connected.
The verb consists of 'zusammen' (together) and 'hängen' (to hang). Literally, it means 'to hang together', which describes both the physical and the logical connection well.
Die Puzzleteile hängen fest zusammen.
The puzzle pieces are firmly connected.
When referring to a logical or causal connection, the preposition 'mit' + dative is almost always used.
Sein Verhalten hängt mit seiner Kindheit zusammen.
His behavior is related to his childhood.
To ask about a connection, the questions 'Wie hängt das zusammen?' (How is that connected?) or 'Womit hängt das zusammen?' (What is that connected to?) are often used.
Womit hängt der plötzliche Erfolg der Firma zusammen?
What is the company's sudden success related to?
to link / connect
Both verbs describe a relationship between circumstances. Verknüpfen refers to establishing a mental or logical connection or, in the passive, to the result of such a connection. Zusammenhängen, by contrast, describes the fact that this relationship exists. Whoever establishes the connection “verknüpft”; the circumstances themselves “hängen zusammen.”

Schlafmangel hängt mit Konzentrationsproblemen zusammen.
Sleep deprivation is related to problems with concentration.
Die Studie verknüpft Schlafmangel mit Konzentrationsproblemen.
The study links sleep deprivation to problems with concentration.