anzweifeln
to question
To express or have doubts about the correctness, truth, or authenticity of something.
Der Anwalt zweifelte die Glaubwürdigkeit des Zeugen an.
The lawyer questioned the credibility of the witness.

to doubt / question
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to question
To express or have doubts about the correctness, truth, or authenticity of something.
Der Anwalt zweifelte die Glaubwürdigkeit des Zeugen an.
The lawyer questioned the credibility of the witness.

'Anzweifeln' (to doubt / question) is a separable verb. The prefix 'an-' is separated from the verb in a main clause and placed at the end of the sentence.
Er zweifelte ihre Geschichte sofort an.
He immediately questioned her story.
This verb always needs a direct object in the accusative. You question 'etwas' (something). This is a fixed grammatical pattern.
Die Opposition zweifelt die neuen Zahlen der Regierung an.
The opposition is questioning the government's new figures.
You typically question abstract things. Common objects are: eine Aussage (a statement), ein Ergebnis (a result), die Glaubwürdigkeit (credibility), eine Theorie (a theory), or eine Entscheidung (a decision).
Niemand wagte es, die Entscheidung des Chefs anzuzweifeln.
No one dared to question the boss's decision.
question
'in Frage stellen' and 'anzweifeln' mean almost the same thing in many situations: you are not sure whether something is correct, true, or valid. 'anzweifeln' is a simple verb and says directly that someone has doubts. 'in Frage stellen' is a fixed expression with 'die Frage' and often sounds a bit more general or formal. It can also express that you no longer simply accept a decision, a rule, or a statement. In ordinary texts, both can often be used in similar ways.

Der Kunde zweifelt an, dass die Rechnung richtig berechnet wurde.
The customer questions whether the invoice was calculated correctly.
Nach dem Fehler stellt das Team die Zahlen im Bericht in Frage.
After the mistake, the team questions the figures in the report.