I have to go.可译成“我得走了.”
I do have to go.则可译成“我确实得走了.”
可以看出,加了do,有腔调的意思.
在用第三人称时,要主要变换：
He has to go.
He does have to go.
即第二句由于加了does,原来的has就要改为have--动词原形.