课(kè) - lesson, class, course in Mandarin

课 (kè) covers a lesson, a class period and a chapter of a textbook. Its measure word is 节 for a timetabled period and 堂 in slightly more formal use: 两节课, two classes.

The verbs pair up neatly. 上课 is to attend class or, for the teacher, to teach it; 下课 is the end of the lesson; 备课 is preparing one; 请假不上课 is taking the time off. Subjects sit in front of it: 中文课, 数学课, 文化课.

The trap is that 上课 is already a verb plus its object, so you cannot hang another object on the end. "I have a Chinese class" is 我有中文课 or 我上中文课, never 我上课中文. Chinese is full of these verb-object pairs, and 上课 is the one learners break first.