
der Unterricht
instruction / class
The teaching of knowledge and skills, usually in a school or another educational institution by a teacher.
Der Deutschunterricht beginnt um acht Uhr.
German class begins at eight o'clock.
instruction / class
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instruction / class
The teaching of knowledge and skills, usually in a school or another educational institution by a teacher.
Der Deutschunterricht beginnt um acht Uhr.
German class begins at eight o'clock.

lesson
In a figurative sense, an experience that represents an instructive lesson.
Diese schwierige Zeit war für ihn ein harter, aber nützlicher Unterricht.
This difficult time was a hard but useful lesson for him.
The noun 'der Unterricht' (instruction/class) is derived directly from the verb 'unterrichten' (to teach), which means 'lehren' (to teach) or 'Wissen vermitteln' (to impart knowledge). The teacher teaches, and what they do is 'der Unterricht' (instruction/class).
Frau Meier unterrichtet seit 20 Jahren an dieser Schule.
Ms. Meier has been teaching at this school for 20 years.
'Unterricht' (instruction/class) is a collective term and is almost always used in the singular, even when talking about many lessons or subjects. You say 'viel Unterricht' (a lot of instruction/class), not 'viele Unterrichte'.
Wegen des Feiertags haben wir morgen keinen Unterricht.
Because of the public holiday, we don't have any class tomorrow.
Common verbs in connection with 'Unterricht' (instruction/class) are 'Unterricht geben' (to teach/give lessons) for teachers and 'Unterricht nehmen' (to take lessons) or 'bekommen' (to get/receive lessons) for learners. 'Am Unterricht teilnehmen' (to participate in class) is also a very common phrase.
Er nimmt privaten Klavierunterricht.
He takes private piano lessons.
To name the school subject, use 'Unterricht in' followed by the dative. When referring to the lesson itself, you say 'im Unterricht' (in class).
Sie ist besonders gut im Unterricht in Biologie.
She is especially good in biology class.
hour
Die Stunde refers to a defined unit in a timetable or course schedule. Der Unterricht is the teaching activity that takes place during this time. Use Stunde for the beginning or duration of the time period; use Unterricht for explaining and learning.

Im Unterricht ab acht Uhr erklärt die Lehrerin den Akkusativ.
In class starting at eight, the teacher explains the accusative.
Die erste Stunde beginnt um acht Uhr und dauert 45 Minuten.
The first class starts at eight o’clock and lasts 45 minutes.