I can't really tell you what's right from wrong.

Or

I couldn't really tell you what's right from wrong.

I can't say what I'm going to do in the future because it's still undecided.

Or

I couldn't say what I'm going to in the future because it's still undecided.

Please tell me the difference between the two?

Thank you.
Why do you continue to ask about can vs could, will vs would, etc, PJ? Please go and check your previous posts on these same topics.
Mister MicawberWhy do you continue to ask about can vs could, will vs would, etc, PJ? Please go and check your previous posts on these same topics.
I really don't understand when it is appropriate to use can or could.

If I'm sure about something I use can, but if there's a slight doubt could is the answer?

I'm not sure.

Please advise.
Students: Are you brave enough to let our tutors analyse your pronunciation?
If I'm sure about something I use can, but if there's a slight doubt could is the answer?-- Yes, that is a good basic guideline!