der Prüfer
examiner
A person who administers an official test or exam and evaluates the candidates' performance.
Der Prüfer bei der Fahrprüfung war sehr streng, aber fair.
The examiner in the driving test was very strict but fair.

examiner / inspector
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examiner
A person who administers an official test or exam and evaluates the candidates' performance.
Der Prüfer bei der Fahrprüfung war sehr streng, aber fair.
The examiner in the driving test was very strict but fair.

The word 'Prüfer' (examiner/inspector) is derived directly from the verb 'prüfen' (to check, to test). The ending '-er' is added to the verb stem to refer to a male person who carries out this activity.
Ein Prüfer ist jemand, der etwas oder jemanden prüft.
A Prüfer is someone who checks or tests something or someone.
'Der Prüfer' belongs to a word family with the verb 'prüfen' (to check, to test) and the noun 'die Prüfung' (the test, the exam). If you know one word, you can understand the others more easily.
Morgen habe ich eine wichtige Prüfung bei einem neuen Prüfer.
Tomorrow I have an important exam with a new examiner.
The exact context determines the type of Prüfer. In a school or university setting, it is someone who tests knowledge (m01). In a company or at a government office, it is someone who checks quality or finances (m02).
Der Prüfer im mündlichen Examen war sehr freundlich.
The examiner in the oral exam was very friendly.
supervisor / caregiver / guardian
'der Betreuer' and 'der Prüfer' can both appear in a school or university context, but they have different tasks. A 'Betreuer' guides a paper or project, gives tips, and provides subject-specific help while someone is writing or studying. A 'Prüfer' evaluates a performance or checks whether something is correct. Learners may confuse the words because both have to do with tasks, exams, or papers. As a simple guide: the 'Betreuer' supports beforehand or during the process; the 'Prüfer' evaluates at the end.

Der Prüfer stellt im mündlichen Examen mehrere Fragen zum Thema.
The examiner asks several questions about the topic in the oral exam.
Mein Betreuer hat mir vor der Abgabe noch wichtige Hinweise gegeben.
My supervisor gave me some important advice before I submitted it.