unter
under
Indicates a position that is spatially lower than something else.
Die Katze schläft unter dem Bett.
The cat is sleeping under the bed.

under
Loading your saved state.
under
Indicates a position that is spatially lower than something else.
Die Katze schläft unter dem Bett.
The cat is sleeping under the bed.

'Unter' is a two-way preposition. You use the dative for a position (question: 'Wo?' (Where?)) and the accusative for movement in a direction (question: 'Wohin?' (Where to?)).
Der Ball liegt unter dem Tisch. (Wo? → Dativ)
The ball is under the table. (Where? → dative)
In most figurative, non-spatial meanings, 'unter' is used with the dative. This applies, for example, to conditions, groups, or numerical information.
Unter diesen Umständen ist das nicht möglich.
Under these circumstances, that is not possible.
As a verb prefix, 'unter-' can be separable or inseparable. The stress determines this: 'úntergehen' (separable, to sink / go down) has the stress on the prefix, while 'unterschréiben' (inseparable, to sign) has the stress on the verb stem.
Die Sonne geht am Horizont unter.
The sun is setting on the horizon.
In the meaning of 'in the midst of a group', 'unter' is often used in fixed expressions. Examples are 'unter uns gesagt' (between you and me / confidentially) or 'unter Freunden' (among friends).
Das bleibt aber unter uns, ja?
This stays between us, okay?
To express a condition, 'unter' is often used. Typical phrases are 'unter der Bedingung, dass…' (on the condition that…), 'unter diesen Umständen' (under these circumstances), or 'unter keinen Umständen' (under no circumstances).
Unter keinen Umständen darfst du diese Tür öffnen.
Under no circumstances may you open this door.
To express that a number, age, or value is not reached, you use 'unter' with the dative. It then means 'less than'.
Der Eintritt ist für Kinder unter 12 Jahren kostenlos.
Admission is free for children under 12 years old.
from / of
'von' and 'unter' can both have to do with a group. 'von' is used when selecting one or more elements from a larger group: 'einer von den Gästen' (one of the guests), 'zwei von den Kindern' (two of the children). 'unter' more often describes that someone or something is in a group or is among others. Learners confuse the words because both can refer to a group. If you are selecting or counting, 'von' fits better. If you mean the position within a group, 'unter' fits better.

Unter den Gästen war auch meine Nachbarin.
My neighbor was also among the guests.
Zwei von den Schülern fehlen heute.
Two of the students are absent today.