1.如果老师对学生们说：“上课”，就是：Class begin，这是个祈使句，所以不加s；2.如果描述课程开始，则要加s，如：class begins on time.
class begin例句
A:When does your class begin?
几点上课？
How does a class begin there?
What time does your class begin? It begins at eight-ten.
你的课几点开始？八点十分开始。
class begins例句
In this term, environmental protection English class begins.
这段教材，就是我们环保英语五十句的第一课。
Test the students before the class begins.
课前先测试学生。
The first class begins at 7:50 am.
第一节课于7点50分开始。