是class is begin还是class begin

是class is begin还是class begin 是before class begin 还是before class is 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 Begin the class Let's begin our class. when dose the class begin? the class is beginning为什么要用beginning而不是begin? ( ) Let,s begin our class Is ____ here now? E( )is here.let us begin our class.