to demand of
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to demand of someone
To demand or require great effort, performance, willingness to sacrifice, or ability from someone.
Der anspruchsvolle Aufstieg hat den Bergsteigern alles abverlangt.
The demanding climb demanded everything of the mountaineers.
'Abverlangen' (to demand of someone) is a separable verb. The prefix 'ab-' comes at the end of the main clause. The verb is often used in the perfect tense with 'haben' (to have).
Diese Prüfung verlangt den Studenten viel Wissen ab.
This exam demands a lot of knowledge of the students.
This verb requires a fixed sentence structure: the person from whom the effort is demanded is always in the dative. The thing that demands the effort is the subject.
Die Pflege ihrer kranken Mutter verlangte ihr viel Geduld ab.
Caring for her sick mother demanded a lot of patience of her.
What is demanded with 'abverlangen' (the accusative object) is often an uncountable noun describing great effort. Common examples are 'alles' (everything), 'viel Kraft' (a lot of strength), 'Geduld' (patience), or 'das Letzte' (the very last bit of strength).
Der Job als Projektmanager verlangt ihm alles ab.
The job as a project manager demands everything of him.
to demand / challenge
Both verbs can express that a task requires a great deal of effort from someone. With „fordern“, the focus is often on the challenging effect of the task; „abverlangen“ places more emphasis on the specific effort or willingness to make sacrifices that is demanded of a person.
Der lange Aufstieg verlangt den unerfahrenen Wanderern viel Ausdauer ab.
The long climb demands a great deal of stamina from the inexperienced hikers.
Der lange Aufstieg fordert die unerfahrenen Wanderer sehr.
The long climb is very challenging for the inexperienced hikers.
