Since I have read that could is past form of can and is used for past abilities but for a specific ability we tend to use “be able to” in affirmative sentences though either can be used in the negatives. Hence my question is that why do we generally use “Could” for a specific ability or a situation in an interrogative sentence for eg: “How could he do that to me?” Instead of “how was he able to do that me” or “How could you say that to me?” intead of “how were you able to say that to me?”. Please guys help me out here. Is it a exception or something else.?
Questions starting with the words: How could.... are generally one of two types. Either they are seeking information:
How could he have escaped?
meaning how was he able to escape,
How could you have passed the exam without studying?
expressing puzzlement and surprise at the result.
Or, they are ways of remonstrating with someone or complaining about someone's actions/behaviour, as in your examples:
How could you say that to me?
How could he do that to me?
Both these examples superficially appear to be asking how someone was able to behave in a manner that was offensive or hurtful, or to be guilty of some harmful/upsetting action.
In fact they serve as accusations that seek an explanation from the person concerned or a sympathetic response from a third party.
The questioner uses this construction to express anger/disillusion/disappointment. The context makes it clear whether its simply information that is being sought - or sympathy, an admission and/or an apology.