hinunter
down
Expresses movement from a higher point to a lower point, away from the speaker. The opposite is 'herunter' (down toward the speaker).
Er ging die Treppe hinunter in den Keller.
He went down the stairs into the basement.

down / downward
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down
Expresses movement from a higher point to a lower point, away from the speaker. The opposite is 'herunter' (down toward the speaker).
Er ging die Treppe hinunter in den Keller.
He went down the stairs into the basement.

The adverb consists of 'hin' and 'unter'. The prefix 'hin-' always describes movement away from the speaker. 'Hinunter' (down) therefore means downward movement that moves away from the speaker.
Schau mal, die Katze klettert den Baum hinunter.
Look, the cat is climbing down the tree.
The counterpart to 'hinunter' (down away from me) is 'herunter' (down toward me). The choice depends on the speaker's perspective.
Bleib oben, ich komme gleich zu dir hinunter.
Stay upstairs; I'll come down to you in a moment.
'Hinunter-' can also be used as a separable prefix for verbs of movement, e.g. 'hinuntergehen' (to go down) or 'hinunterfallen' (to fall down). In a main clause, the prefix stands at the end.
Wir gehen jetzt die Treppe zum Ausgang hinunter.
We are now going down the stairs to the exit.
In the meaning of swallowing, 'hinunter' (down) is almost always combined with verbs such as 'schlucken' (to swallow), 'würgen' (to choke/gag), or 'spülen' (to wash down). It emphasizes the movement through the throat.
Das Kind wollte die bittere Medizin nicht hinunterschlucken.
The child did not want to swallow the bitter medicine down.
above / up
'oben' and 'hinunter' are opposites because they are about height and direction. 'oben' describes a higher position, or with 'nach oben' movement toward a higher place. 'hinunter' describes movement from a higher point to a lower point, usually away from the speaker. Learners may confuse them because both are used with stairs, paths, or mountains. Pay attention: 'oben' is a place or destination up high; 'hinunter' is the movement downward.

Die Kundin ist die Treppe hinunter zur Kasse gegangen.
The customer went down the stairs to the checkout.
Nach der Arbeit stellt er die Kiste oben auf den Schrank.
After work, he puts the box on top of the cupboard.