
sich unterhalten
to converse
To have a conversation with one or more people; to talk with each other.
Wir haben uns auf der Party stundenlang über Musik unterhalten.
We conversed about music for hours at the party.
to converse / entertain / maintain
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to converse
To have a conversation with one or more people; to talk with each other.
Wir haben uns auf der Party stundenlang über Musik unterhalten.
We conversed about music for hours at the party.

to entertain
To provide someone with pleasant diversion or enjoyment; to amuse someone.
Der Komiker hat das Publikum den ganzen Abend lang bestens unterhalten.
The comedian entertained the audience superbly all evening.

to maintain
To care for something and keep it in good condition, e.g. a building, a garden, or also relationships.
Die Firma unterhält seit vielen Jahren gute Geschäftsbeziehungen zu Kunden in Asien.
The company has maintained good business relationships with customers in Asia for many years.

to support
To provide for someone's livelihood; to support someone financially.
Er muss mit seinem Gehalt eine vierköpfige Familie unterhalten.
He has to support a family of four on his salary.
'Unterhalten' is an inseparable verb. The prefix 'unter-' remains firmly attached to the verb 'halten' (to hold) in all tenses and sentence types. The stress is on the second syllable: unterHAlten.
Er hat seine Gäste den ganzen Abend gut unterhalten.
He entertained his guests well all evening.
Using the reflexive pronoun 'sich' changes the meaning completely. 'Sich unterhalten' always means 'to talk with each other'. Without 'sich', the verb has other meanings such as 'to entertain', 'to maintain', or 'to support financially'.
Die beiden Nachbarn unterhalten sich jeden Morgen über den Gartenzaun.
The two neighbors converse every morning over the garden fence.
When you converse, you use the preposition 'mit' + dative for the person and 'über' + accusative for the topic of the conversation.
Ich habe mich lange mit meiner Freundin über unsere Zukunftspläne unterhalten.
I had a long conversation with my friend about our plans for the future.
In the meaning of 'to maintain', the verb is often used with abstract nouns. Typical combinations are 'Geschäftsbeziehungen unterhalten' (to maintain business relationships), 'Kontakte unterhalten' (to maintain contacts), or 'ein Netzwerk unterhalten' (to maintain a network).
Es ist wichtig, gute Beziehungen zu den Nachbarn zu unterhalten.
It is important to maintain good relationships with the neighbors.
The meaning 'to support financially' is always used in the context of money and living expenses. Typical objects are 'eine Familie' (a family), 'ein Kind' (a child), or also 'sich selbst' (oneself).
Sie muss mit ihrem Teilzeitjob zwei Kinder unterhalten.
She has to support two children with her part-time job.
to receive / preserve / support
When referring to a building, erhalten and unterhalten can both concern keeping it in good condition. Erhalten emphasizes the goal of securing its existing condition or preserving the building itself. Unterhalten focuses more on the ongoing care and regular work required to do so.

Die Stadt unterhält das alte Rathaus mit regelmäßigen Reparaturen.
The city maintains the old town hall with regular repairs.
Die Stadt erhält das alte Rathaus für kommende Generationen.
The city is preserving the old town hall for future generations.