I'd like to know when it's appropriate to use "können" and when "könnten", are they completely interchangeable? Is it like the difference between "can" and "could" in English?
Können Sie mir bitte sagen wie spät es ist?
Könnten Sie mir sagen wie viel Uhr es ist, bitte?
I found plenty of examples on Google but they didn't seem reliable to me.
Yes, it's pretty much the same difference as "can" and "could" in English.
In this specific case both words do a very similar thing, in that they both turn a direct request into a more polite question.
"Könnten" is the subjunctive (in German: "Konjunktiv") of "können". One of the usages of Konjunktiv is expressing courtesy. As explained in this article "könnten" is the even more polite, humble form.
So these three sentences have technically the same goal:
"Sagen Sie mir bitte wie spät es ist!"
is very direct, almost rude. It's a direct requests and could be considered an order, if it weren't for the word "bitte".
"Können Sie mir bitte sagen wie spät es ist?"
is already more polite and softens the "demand" by asking if they could do this for me.
"Könnten Sie mir sagen wie viel Uhr es ist, bitte?"
is the most polite form of the three. It's effectively saying "I'm not entitled to this action, but I'm still asking you to do it, if you would be willing to".
In another context, they choice between the two words could change the actual meaning as opposed to just bein a more polite form of the same, however:
The first one indicates that we are ready to go now. The second one indicates that we could go now, but there's something that's stopping us (even if it's just the decision to actually go).
Yes, the other answer is correct; können means can and könnten means could.
What is important though is that in your given example könnten should be used when you would like to be more polite. By putting your verb into the subjunctive for a question it becomes a bit more courteous: Könnten Sie mir bitte sagen...
