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.

'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
'erhalten' and 'unterhalten' overlap when it comes to keeping something in existence. 'erhalten' means to preserve something or keep it in good condition. 'unterhalten' can also mean that, but it often sounds like regular maintenance or costs, for example with a house, a garden, or a relationship. It can also mean to support someone financially. This meaning is close to 'sich erhalten', but the direction is different: you support yourself, but you 'unterhalten' another person. Learners should also watch out because 'sich unterhalten' usually just means 'to talk with one another'.

Nach der Trennung unterhält Paul seine zwei Kinder.
After the separation, Paul provides for his two children.
Mit viel Arbeit hat die Familie das alte Haus erhalten.
With a lot of work, the family preserved the old house.