He is having a bath为什么要用having而不用doing

he is having a bath He is having a good time.为什么用having而不用hasing . he has a bath he is having a bath he had a bath He's having a bath 请问have已经是动车了,is可以直接去掉吗（he having a bath)为 having shower uses less water than having a bath.为什么用ing形式 It must be teenages having fun 这里为什么要用having 而不用to have It must be teenagers having fun 为什么这里要用having而不用to have 书上的是he's having a bath .为什么不是这样he's bathing 为什么he is having lunch at home now.中的having不用hasing? ( )He went to bed ____ having a bath.选什么 —Your dog is having a bath again. The rain having stopped,he went out for a walk.为什么要用having?