that must have been a long trip为什么要加have什么作用不用不行么?

-That must have been a long trip to get there - Yeah ,it____ How long have you been 为什么加suffering from a headache不用suffer "i have been for a long time"中,"have" 为什么不用"had"? we have been best friends for a long time中friends为什么要加s take a look at that car,it must have been expensive,为什么用have We have been good friends a long time.这里的a long time之前不用加for How long have you been dancing today?为什么要加been!而T He must have been dreaming为什么加ing have been long 什么意思 We have been waiting a long time.不用for a long time It's+been++a+long+time.been在这里起什么作用?为什么要用been how long have you been studying为什么后面要加ing