'Yet' and 'already' are often used with Present Perfect.
Which of them is preferred in sentences like this:
Is your father here yet (or already?)? Yes, he has just come.
'Yet' is used to say that it is expected that something has happened and used in negative sentences and in questions, so in the sentence above it seems that it is used correctly.
But is it possible to use already in this question? Would already sound naturally here?
The difference is illustrated by imagining a blind passenger asking the car driver...
1: "Are we there yet?"
2: "Are we there already?"
...where the different implications are...
(1) it's [already] been a long journey, and I hope we've finally arrived [we should have].
(2) it's not been a long journey [yet], but surprisingly it seems we might already have arrived.
To put it another way, in such contexts, yet = after so much time, already = after so little time. That's to say, use yet if you were expecting/hoping for earlier, or already if you were expecting later (but not yet).
Note that both words can be used in contexts where either or both of expectation/likelihood and hope/desire are involved. In practice it makes no difference if an impatient child on a long car journey asks "Are we there yet?" or "Aren't we there yet?". But in other contexts, framing such questions in the negative often comes across as rude/impatient/accusatory, rather than a neutral request for information.
While both are grammatically valid sentences they mean different things, and yet is surely what you'd want to use in this case.
Person A: Is your father here yet?
Person B: Yes, he has just arrived.
(Note the use of arrived rather than come; come needs an additional "from where", ex. "Yes, he has just come home from work.")
When using yet here, you're asking a question: is your father here. You don't know the answer, and you expect a yes or no reply.
It's a bit different when you use already:
Person A: sees or hears something that makes them realize the father is here, but they didn't expect the father to arrive until later
Person A: Is your father here already?
In this case the question is a rhetorical one; Person A is aware that the father is there, but is surprised by this fact. So they state that the father is there already, as opposed to later on. They don't expect an answer, because they already know. This is just a way of expressing surprise.
