
darunter
among them
Indicates that something or someone is in a group that was mentioned previously; among others, included.
Es kamen viele Gäste, darunter auch einige alte Schulfreunde.
Many guests came, among them several old school friends.
among them / underneath
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among them
Indicates that something or someone is in a group that was mentioned previously; among others, included.
Es kamen viele Gäste, darunter auch einige alte Schulfreunde.
Many guests came, among them several old school friends.

underneath
Describes a position below something else.
Er legte das Buch auf den Tisch und schob seine Brille darunter.
He put the book on the table and slid his glasses underneath.

below that
Refers to a value or amount that is lower than a stated limit.
Die meisten Wohnungen kosten über 1000 Euro, es gibt nur wenige Angebote darunter.
Most apartments cost over 1,000 euros; there are only a few offers below that.

from it
Used with verbs like 'leiden' (to suffer) to refer to the cause of a negative feeling or condition.
Der Lärm war unerträglich und die Anwohner litten sehr darunter.
The noise was unbearable, and the residents suffered greatly from it.
'Darunter' is a pronominal adverb made up of 'da(r)' and the preposition 'unter' (under). 'Da(r)' refers to something that was previously mentioned in the sentence or context.
Auf dem Tisch liegen viele Bücher, darunter mein Lieblingsroman.
There are many books on the table, among them my favorite novel.
The context shows the exact meaning: If 'darunter' refers to a group, it means 'included/among them'. If it refers to a place, it means 'underneath'. If it refers to a number, it means 'less than/below that'.
Es gibt viele Apfelsorten, darunter süße und saure.
There are many apple varieties, among them sweet and sour ones.
In the meaning of 'eingeschlossen' (included), 'darunter' often introduces a subordinate clause or an apposition to give examples from a previously mentioned group. It then usually comes after a comma.
Viele europäische Städte sind sehenswert, darunter Paris, Rom und Prag.
Many European cities are worth seeing, among them Paris, Rome, and Prague.
The combination 'darunter leiden' (to suffer from it) is a fixed expression. 'Darunter' refers to the cause of the suffering, which was mentioned earlier in the conversation or text.
Die ständige Kritik verletzte sie, und sie litt sehr darunter.
The constant criticism hurt her, and she suffered greatly from it.
under
Unter names the lower position together with its reference, for example unter dem Fenster. Darunter denotes the same position when the reference has already been mentioned or is clear from context. Use unter with the reference and darunter as a reference back to it.

Das Fenster ist oben, und das Regal hängt darunter.
The window is above, and the shelf is mounted underneath.
Das Regal hängt unter dem Fenster.
The shelf is mounted under the window.