eifrig
eager
Being involved in something with great diligence, commitment, and enthusiasm.
Die Studenten machten sich eifrig Notizen während des Vortrags.
The students eagerly took notes during the lecture.

eager / diligent
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eager
Being involved in something with great diligence, commitment, and enthusiasm.
Die Studenten machten sich eifrig Notizen während des Vortrags.
The students eagerly took notes during the lecture.

The adjective 'eifrig' (eager/diligent) belongs to the noun 'der Eifer' (zeal), which means 'großer Fleiß' (great diligence) or 'Begeisterung' (enthusiasm). So someone who is 'eifrig' shows a lot of 'Eifer'.
Mit großem Eifer begann sie, die neue Sprache zu lernen.
With great enthusiasm, she began learning the new language.
'Eifrig' (eagerly/diligently) is very often used as an adverb to describe the manner of an action. It then usually comes directly after the conjugated verb.
Die Kinder bastelten eifrig an ihren Geschenken.
The children eagerly worked on their gifts.
The word often describes activities that require concentration and diligence. Typical verbs are 'eifrig lernen' (to study eagerly), 'eifrig arbeiten' (to work diligently), 'eifrig lesen' (to read eagerly), or 'eifrig nicken' (to nod eagerly).
Er ist ein eifriger Schüler und macht immer seine Hausaufgaben.
He is a diligent student and always does his homework.
hardworking
'fleißig' and 'eifrig' both describe someone being active and committed to something. 'fleißig' mainly emphasizes work, effort, and persistence. 'eifrig' additionally emphasizes enthusiasm or a strong desire to do something. The words can be confused because both often fit learning, work, or helping. If someone regularly works a lot, 'fleißig' is more natural. If someone gets started right away with a lot of energy and enthusiasm, 'eifrig' fits better.

Jonas hat im Unterricht eifrig Fragen gestellt, weil ihn das Thema interessiert hat.
Jonas eagerly asked questions in class because he was interested in the topic.
Sie arbeitet im Supermarkt fleißig, auch wenn viel los ist.
She works hard at the supermarket, even when it is busy.