Your friend's hair is much shorter than when you last met.What do you say to her?
问题描述：
Your friend's hair is much shorter than when you last met.What do you say to her?
为什么是：You had your hair cut?
不是：You have your hair cut?
答
因为have your hair cut “理发”该动作在问话的时候已经发生了啊,否则问她这个问题干嘛啊?
既然该动作已经发生,所以此句时态当然使用一般过去时,而不能使用一般现在时了.