When someone asks me
Can you do this?
I feel that it is missing the “please”.
Is “please” already implied by “can”, or is it proper to ask
Can you please do this?
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The word "can" does not imply please. It is nice to say please but please does not always mean someone is being polite.
Usually the word "please" might be added to a simple sentence like that because the person asking may feel like you are not doing X fast enough, that you normally wouldn't do X, that X is doing them a favor... I personally like not hearing the please because it usually means that everything is running smoothly. I would hate to think that someone would feel they need to say please to get me to do something.
Fundamentally, "can you please do..." is a request, while "can you do..." is a question (albeit one with an implied request attached).
Can you pass the salt?
Literally, this is asking whether the salt shaker is within your reach, and thus whether you have the capability to grab it and pass it along. However, there is also an implied request, namely if you are capable of doing it, then you should do so. This does not mean that there's an implied "please" in there; this is definitely the lowest on the politeness scale.
Can you please pass the salt?
This is no longer asking about capability; you are assumed to be able to reach the salt shaker and pick it up. As a request, it's polite enough for most purposes, but we can do better.
Could you pass the salt?
This is also an outright request; capability is assumed. Where it belongs on the politeness scale is somewhat debatable, but I would say it's about equal to "can you please...".
Could you please pass the salt?
This is the proper, polite form of this request. We have both the conditional and the "please" in there, so the request has been softened about as far as it can be without resorting to overblown false humility ("I was wondering if I could possibly trouble you to do me the immense favor of please passing the salt").
That all said, the politeness (or lack thereof) of a request depends greatly on the tone. "Can you pass the salt?" said without any particular emphasis would be much more polite than "Can you please pass the salt?!", because the latter, with the emphasis on the please and the exclamatory tone, implies frustration rather than politeness.
To address something from the comments:
Could you please just pass the salt?
Here, we've blown right past politeness and into exasperation. This would only be used if you've already asked for the salt, and instead of being passed the requested spice container, you were given a long story about how salt is bad for you and how it's impolite to ruin the spicing that the chef has so carefully achieved and really, don't you think the stew is already salty enough?
Both are proper to ask. They both convey the same kind of request. The first is more direct and can be used in normal day to day business, casual, etc. conversation. The second is a little more polite but caries the same meaning. If I were speaking to my dad, I'd use the second. If I were speaking to my office college or a friend, I'd use the first. That said, please is not implied in the first.