herab
down
Expresses movement from a higher to a lower position; downward.
Vorsichtig stieg er von der Leiter herab.
He carefully climbed down from the ladder.

down
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down
Expresses movement from a higher to a lower position; downward.
Vorsichtig stieg er von der Leiter herab.
He carefully climbed down from the ladder.

The word consists of two parts: 'her-' and 'ab'. The prefix 'her-' expresses movement toward the speaker, while 'ab' describes movement downward.
Der Apfel fiel vom Baum herab.
The apple fell down from the tree.
'Herab' (down) is very often the separable prefix of a verb, for example in 'herabsteigen' (to climb down/descend) or 'herabblicken' (to look down). In a main clause, the prefix stands at the end.
Er blickte vom Balkon auf die Straße herab.
He looked down from the balcony onto the street.
The word 'herab' (down) is used more often in written language or in a more formal style. It can sound slightly elevated or literary.
Schnee fiel leise auf die Landschaft herab.
Snow fell softly down onto the landscape.
The figurative, contemptuous meaning is almost always used in the fixed phrase 'von oben herab' (condescendingly/from above). Typical verbs in this context are 'behandeln' (to treat), 'ansehen' (to look at/view), or 'reden' (to speak/talk).
Sie behandelt ihre Mitarbeiter oft von oben herab.
She often treats her employees condescendingly.
down
'runter' and 'herab' can both express movement from up high to down below. The most important difference is style: 'runter' is colloquial and fits well in everyday conversations. 'herab' sounds more formal, more written, or somewhat more elevated. In many everyday situations, people are more likely to say 'runter'. 'herab' is used more in texts or when something should sound more precise or more solemn. The words can be confused because the direction is the same. But pay attention to the tone of the situation.

Von der alten Brücke fiel ein Stein herab.
A stone fell down from the old bridge.
Komm bitte kurz runter, das Essen ist fertig.
Please come down for a moment; dinner is ready.