When do people buy new clothes 不是when is吗？没关系，就算你不回，我也会采纳你。你挺认真的。你快点回啊！求求你啦！此句的动词是一般现在时buy,变成问句时当然要用助动词do啦! 此句又是问时间,因此答案就是When do people buy...?

不能用is的,因为be动词(如am,is,are,was,were)一般用于主系表结构或构成进行时态、被动语态。如：She is a student .
He is watching TV at home.
Football is played all over the world.