Can I use these interchangeably?
(something new, a negative state etc.)

I still can't get used to it.
I still couldn't get used to it.

Question

Can I use these interchangeably?
(something new, a negative state etc.)

I still can't get used to it.
I still couldn't get used to it.

Answer

No.
can talks about current possibility, whereas could talks about past possibility.
example:
1). no matter what happens I can not do 'X'.
2). no matter what happened I could not do 'X'.