No rule for this one
No ending or word group predicts the article of Formular. Learn it as a pair and say it out loud: das Formular.
das Formular
It's das Formular. Formular is neuter, plural die Formulare.
Bitte füllen Sie das Formular aus.
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No ending or word group predicts the article of Formular. Learn it as a pair and say it out loud: das Formular.