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
'fordern' and 'abverlangen' can both mean that something requires a lot of effort or strength. 'fordern' is more general: a task, training, or situation can require attention, skill, or effort. 'abverlangen' is used especially when someone is expected to cope with something difficult or burdensome. It almost always appears with a person in the dative: 'jemandem etwas abverlangen' (to demand something of someone). Learners may confuse the words because both have to do with effort. As a practical rule: 'fordern' is broadly applicable, while 'abverlangen' emphasizes more strongly that a person has to achieve or endure a lot.
Die lange Wanderung hat den Kindern viel abverlangt.
The long hike demanded a lot of the children.
Die neue Stelle fordert viel Geduld und gute Planung.
The new position requires a lot of patience and good planning.