@JTT,
Oh... I must also admit today I don't feel quite well and when I look at my posts I don't understand why I made such a mistake. Sorry~
To put my question right:
THAT must be in the past, then why doesn't that person say:
'How COULD you say that?'
COULD here is the usual past form of CAN; in modern English, to refer to the present, we can use CAN/COULD; to the past, only COULD is possible, when the meaning is ABLE/IN A POSITION TO DO STH; in that conversation, the speaker is referring to a particular thing said in the PAST, so I am not able to use grammar to explain it, though I think it's natural....