是before class begin 还是before class is begin

是before class begin 还是before class is begin 是class is begin还是class begin 是Class begin.还是Class begins.还是Classes begin.为什么? "class begin"还是"classes begin"? 是用Class begin 还是Class begins 是class starts 还是class begin I didn't begin the class before all the students stoped talk Begin the class 英语翻译ending before i begin Let's begin our class. when dose the class begin? the class is beginning为什么要用beginning而不是begin?