Could / Can Happiness bring pain?
The above sentence is not meant to mean the past time. "Could", and not "can", is used for past. So, "can" is preferable in the sentence?
On the other hand, the sentence is meant to ask whether there are possible conditions under which happiness bring pain. Or, does it ever happen that happiness brings pain. So, since it is about future possibility, isn't "could" a better choice?
Can and could are both modal auxiliaries and they both refer to possibility and probability.
So do the other modal auxiliaries.
All of the following questions (which are all grammatical) ask
"whether there are possible conditions under which happiness brings pain".
The first verb in each case is a modal auxiliary verb. None of them refer to the past.
Forget about "past tense" with modals. Modal auxiliaries are uninflected and have no tense at all.
Could only refers to past time in specialized constructions, which are rare.
It does not usually refer to the past.
The same applies to the other "past" modals: would, might, should, must.
They refer much more often to the future, in fact.
Can and could are both fine here, though I would recommend adding ever to clarify the intended meaning (unless other context already makes that clear):
The difference between the two is that can seems to be asking about whether it ever really happens, whereas could seems to be asking about whether it could even hypothetically happen. (Neither of these is strict, however; we can say things like, on the one hand, "Is there any possible world where happiness can ever bring pain?", and, on the other hand, "Do you think that, in the average person's life, happiness could ever bring pain?")
[…] the sentence is meant to ask whether there are possible conditions under which happiness bring pain. Or, does it ever happen that happiness brings pain.
But those aren't the same! For the former, I'd recommend could; for the latter, I'd recommend can (or does).
