they usually play football (after class)对括号部分进行提问
问题描述：
they usually play football (after class)对括号部分进行提问
答
when do they usually play football?
答
When do they usually play football?
after class是时间状语，对时间状语提问用when
答
When do they usually play football?
答
When do they usually play football? (after class)是一种状态，可以被认定为时间，即用when 提问