
hinab
downward
Expresses movement from a higher point to a lower point, typically away from the speaker. It describes the direction downward.
Er stieg vorsichtig die steile Felswand hinab.
He carefully climbed down the steep rock face.
down / downward
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downward
Expresses movement from a higher point to a lower point, typically away from the speaker. It describes the direction downward.
Er stieg vorsichtig die steile Felswand hinab.
He carefully climbed down the steep rock face.

downhill
Used figuratively for a descent, a deterioration, or a decline.
Seit der neuen Geschäftsführung ging es mit der Firma stetig hinab.
Since the new management took over, the company has steadily gone downhill.
The word is made up of 'hin' (away/toward there) and 'ab' (down/off). 'Hin' describes movement away from the speaker, and 'ab' means downward. So 'hinab' means movement downward, away from the speaker.
Vom Berggipfel blickten wir auf das Dorf tief unter uns hinab.
From the mountain summit, we looked down at the village far below us.
'Hinab' can be used as an independent adverb or as the separable prefix of a verb. As a prefix, it appears at the end of the main clause.
Der Weg führt steil den Berg hinab.
The path leads steeply down the mountain.
'Hinab' belongs more to written language or to an elevated, more formal style. In everyday colloquial speech, other expressions are often preferred.
Die Sonne versank und Dunkelheit senkte sich auf das Tal hinab.
The sun sank, and darkness descended upon the valley.
The literal meaning refers to a physical movement downward, for example down stairs or a mountain. The figurative meaning describes a negative development in abstract things, for example a company or quality.
Er kletterte die Leiter hinab.
He climbed down the ladder.
In its literal meaning, 'hinab' is often combined with verbs of movement. Typical ones are: 'hinabsteigen' (to descend), 'hinabgehen' (to go down), 'hinabklettern' (to climb down), 'hinabsehen' (to look down), or 'hinabgleiten' (to glide down).
Der Taucher glitt langsam in die Tiefe hinab.
The diver slowly glided down into the depths.
down
Herunter and hinab both describe movement from above to below. Herunter is the neutral everyday form and can indicate movement toward the speaker; hinab tends to emphasize movement away from a reference point and often sounds more elevated. In normal conversation, herunter is usually the better choice, while hinab is common in narrative language.

Der Hund lief die Treppe zum Hof hinab.
The dog ran down the stairs to the courtyard.
Der Hund lief die Treppe zu mir herunter.
The dog ran down the stairs toward me.