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Can you please explain this to me?
Could you please explain this to me?
I am unable to figure out which to use which situation. I did google, and some posts say they are both the same, even if the second one is more formal. Other posts say could is the past tense of can, but in the above example, I don't think could is used as past tense of can.
What is correct way to use these words?
The quick answer: In most contexts, these two words are effectively synonyms, especially when discussing possibility, e.g. "(Can/Could) you [perform some action]?".
Longer answer:
There is a somewhat subtle difference in these two phrases that becomes a lot less subtle in different contexts. Let's take the words one at a time:
Can deals with actual, literal ability to perform an action. When you ask if someone can explain something, you are literally asking whether they are capable of explaining it. Edit: It is important to note that, as @J.R. has pointed out, this is by no means the only use (or even the most common use) of the word can or the phrase can you .... This construction is often abused to mean effectively the same thing as could you..., i.e. would you be willing to..., etc.
Could deals with the possibility of performing an action. It is definitely a subtle difference, but basically, when you discuss possibility, you generally are aware of their ability to do it but are tentative about their willingness or availability to perform the action. This is by no means an exact definition.
It is also worth noting that the word could is actually the past tense of can. In English, using the past tense form of a word in this way is often meant to indicate tentativeness.
I wanted to talk with you.
I wondered if you would be available this afternoon.
I thought it might a good idea.
In OP's context, the only difference is that could/would you [do something]? is slightly more polite and/or "deferential" than can/will you [do it]?
The reason for this is simply could/would (and might) are verb forms more strongly associated with hypothetical scenarios. So using them places more "distance" between the speaker and whatever he's asking for. There's more on the issue in this question on ELU, and if you're prepared to read a bit more, have a look at this excellent answer to a closely-related question.
Re OP's point about could being a "past tense", I'd say that from some perspectives English doesn't really have a "past tense" anyway. What it has, particularly with modal verbs like can, may, must, shall, will is verb forms that often refer to the past, but can also refer to anything that's not here, now, present.
Thus if I say "I could do it yesterday, and I can still do it today", could references past time (before the present) and can references the present. But if I say "But I couldn't do it again without help", could [not] references either a future time (after the present), or a hypothetical time ("away" from the present), depending on how you want to look at it.
Could can be more tentative, and may sometimes be more polite. However, the meaning depends on the context, the relationship between the participants in the conversation, and the speaker’s intonation.
Because could is a form of can, the sensible differences between those requests come from tense/mood.
Can inquires about the present, definite future, or general ability/possibility:
Could indicates the subjunctive:
If we mean to use can/could politely we're using them in pointed contrast to literal meaning: we're politely interested in convenience, preference, etc., and for that reason we do not ask about those things, and allow the conceit of ability to determine the outcome. And also asking after ability allows someone to issue a more definite "no" than asking after preference, convenience, whch is important.
Can you please explain this to me? Could you please explain this to me?
Answer: Can is a rude way to ask, it's like giving a command. Could is a polite way to ask, it's like a requesting.
For this case, "Could you please explain this to me"? is the suitable option.
