The modal auxiliary could is used
in several very different
ways. One happens when
sentences with can are used in
(reported) speech and the main verb is past.
Bill said, "Can you help
me, Carol?" ----->
Bill asked Carol if
she could help him.
"I can't now, but I can later." ----->
replied that she couldn't help Bill then
added that she could help him later.
Lucy said, "Can you speak Thai,
Lucy asked Dave if he could speak Thai.
Dave replied, "I can understand
it better than
I can speak it. ----->
Dave answered that he could understand Thai
better than he could speak it.
I have some dessert, Mom?" ----->
asked his mother if he could have
said, "No, you can't, Bobby,
haven't finished your vegetables. ----->
Bobby's mother told him that
he couldn't have
any dessert because
he hadn't finished his vegetables.
||The time for can
is still present or future in the
sentences above. In the sentences,
to could "artificially" because
main verb is past.
In an indirect-speech speech
sentence, it's possible
not to change the verb
after can if it refers to
that is in general time:
Dave said, "I can understand
Thai better than
I can speak it. -----> Dave
said that he can
understand Thai better
than he can speak it.
(The sentence is also correct
||The negative of could in
sentences such as
the sentences above is couldn't (or could not).