
ungenügend
insufficient
Not adequate in quantity, quality, or degree; not meeting the requirements.
Die Beweise waren für eine Verurteilung ungenügend.
The evidence was insufficient for a conviction.
insufficient / failing
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insufficient
Not adequate in quantity, quality, or degree; not meeting the requirements.
Die Beweise waren für eine Verurteilung ungenügend.
The evidence was insufficient for a conviction.

failing
The worst school grade, grade 6, in the German grading system, used to assess a completely inadequate performance.
Er hat die Prüfung mit „ungenügend“ nicht bestanden.
He failed the exam with a grade of 'ungenügend' (failing).
The word is built simply: the prefix 'un-' negates the word 'genügend' (sufficient). 'Ungenügend' (insufficient) therefore literally means 'not sufficient.'
Die Beleuchtung im Raum war für die feine Arbeit ungenügend.
The lighting in the room was insufficient for the delicate work.
'Ungenügend' (insufficiently) can stand not only as an adjective before a noun, but also as an adverb describing a verb. It then means 'in an insufficient way.'
Die Mannschaft war ungenügend auf das wichtige Spiel vorbereitet.
The team was insufficiently prepared for the important game.
In the German school system, 'ungenügend' (failing) is the fixed term for the worst grade, grade 6. In this context, it is not a general description but a defined technical term.
Sein Test wurde mit „ungenügend“ bewertet, weil er keine einzige Frage beantwortet hat.
His test was graded 'ungenügend' (failing) because he did not answer a single question.
When the word is used as a noun, 'ein Ungenügend' (a failing grade), it almost always refers to the school grade 6. For example, you can say 'ein Ungenügend bekommen' (to get a failing grade) or 'im Zeugnis ein Ungenügend haben' (to have a failing grade on one’s report card).
Für diesen Aufsatz hat sie leider ein Ungenügend bekommen.
Unfortunately, she got a failing grade for this essay.
insufficient
Unzureichend and ungenügend both describe performance that fails to meet requirements. Unzureichend states this deficiency in general terms. Ungenügend sounds more like an evaluation and can specifically refer to the worst school grade in Germany.

Der Lehrer bewertete die Leistung wegen der fehlenden Aufgabenteile mit ungenügend.
Because parts of the task were missing, the teacher gave the performance a failing grade.
Die Leistung war unzureichend, weil wichtige Teile der Aufgabe fehlten.
The performance was insufficient because important parts of the task were missing.