instand setzen
to repair
To repair or restore something so that it is once again in good, functional condition.
Die alte Brücke muss dringend instand gesetzt werden.
The old bridge urgently needs to be repaired.

repair / maintain
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to repair
To repair or restore something so that it is once again in good, functional condition.
Die alte Brücke muss dringend instand gesetzt werden.
The old bridge urgently needs to be repaired.

The word 'instand' is used almost exclusively in the fixed combinations 'instand setzen' (to repair) and 'instand halten' (to maintain). It does not function as an independent adjective.
Die Firma wurde beauftragt, die Heizungsanlage instand zu setzen.
The company was commissioned to repair the heating system.
'Instand setzen' (to repair) describes a one-time action to make something broken functional again. 'Instand halten' (to maintain) describes a regular activity to preserve good condition.
Nach dem Sturm musste das Dach instand gesetzt werden.
After the storm, the roof had to be repaired.
The expressions with 'instand' are typically used for technical devices, buildings, vehicles, or facilities. For example, one speaks of repairing or maintaining a machine, a house, or a road.
Die historischen Maschinen im Museum werden sorgfältig instand gehalten.
The historic machines in the museum are carefully maintained.
The expressions 'instand setzen' (to repair) and 'instand halten' (to maintain) belong to a rather formal or technical style of language. They are often found in contracts, instructions, or official reports.
Laut Mietvertrag ist der Mieter verpflichtet, die Wohnung instand zu halten.
According to the rental agreement, the tenant is required to maintain the apartment.
to receive / preserve / support
'erhalten' and 'instand' are both connected to something being in good condition. 'erhalten' generally means that something is preserved and does not break, decay, or disappear. 'instand' usually appears in fixed expressions: 'instand halten' means to keep something in good condition through maintenance, while 'instand setzen' means to repair something. Learners may confuse the words because both can fit with buildings, devices, or systems. 'erhalten' is broader and can also be used for culture or nature. 'instand halten' and 'instand setzen' are especially suitable for things that need to function.

Der Hausmeister hält die Heizung regelmäßig instand.
The caretaker regularly maintains the heating system.
Die Familie möchte das alte Haus in seinem ursprünglichen Zustand erhalten.
The family wants to preserve the old house in its original condition.