My girlfriend always says that I am wrong when I text her to say "I'm there". Is it correct to say "I'm here" or "I'm there" when I arrive at her house? Also why is that the correct way to state my arrival?
Everyone I can think of would normally use 'I'm there' to announce his arrival… certainly in speech and very likely in writing though it's hard to imagine a rule of grammar covering it.
Consider the kids in the back of the car, constantly whining 'Are we there yet?' They mean 'Are we yet at the place we're going to,' don't they?After minutes, let alone hours or days on the road, they think of the destination as 'there' and there's a clue, even though it's obscure. 'After' is the key word here.
For the duration of the journey, the destination is 'there'. After the end of the journey, the same place is no longer a destination; it's become their current location.
The question is, at what point does the journey end? Fairly clearly, arriving early or late can make no difference so that point is not in time but in space.
Consider being half a mile away from the destination. Do I ring you and ask 'Can you get the drinks ready for when I get here?' or '… there?' Try the same question 100 yards out.
Try it 25 feet from the door… and less… At some point it will no longer be 'When I get…' at all. It will have become 'Now that I'm here, have you got the drinks ready?' or 'I'm here. Have you got the drinks ready?'
Roughly, while I'm travelling to your house, I'm going 'there'. Of course from your point of view that’s ‘here’ but who's doing the talking? Until the moment I step over the threshold, I'm still going 'there.' Only after I've actually walked through the door and entered the house am I 'here'.
Whether it’s the threshold or the porch or the steps or the path or the lawn is rather subjective and either I’m travelling ‘there’ or I’ve arrived ‘here’.
The difference between speech and writing, oddly, is simply that it’s a lot difficult to write while still on the move. To be able to write a letter home saying ‘Now that I’m here, I’m sure things will be just wonderful…’ requires that I’m physically sitting at a desk, or some such. That necessarily means the journey is over, else how could I have started to do things which require being comfortably somewhere, rather than travelling.
I might write “I always wanted to visit Ruritania and now that I’m there, blah lah lah’ and I might as easilyt say “… now that I’m here…"
Even if that wasn't just an introductory preamble, it would be subjugating meaning to strictly matched tense. The next sentence; the first real sentence, is going to say ‘I’m staying at the Hotel Splendide and while I’m here, I want to visit the museum and the bazaar’ is it not?
Your girlfriend is right. If you are meeting her somewhere: at her house, at a theatre, at the mall—when you arrive at the designated spot, you say "I'm here." This is true whether or not she is already at that spot. If she's home and you're outside her house to pick her up for a date, you say "I'm here." If you're meeting her at the movies and you arrive before she does, you text "I'm here."
Few circumstances would call for "I'm there", and they all assume your girlfriend is not at the same spot, nor going to be. For example, if she were on the phone explaining to you where something is stored in her house and giving you directions to that exact spot, you might say "I'm there". Here's a hypothetical text exchange. Suppose she is not home, but urgently needs something that she left behind there, and wants you to pick it up for her. She starts off:
"I need your help! Can you go over to my house right now?"
[you agree, and drive over]
"I'm here. Now what?"
"Go to the kitchen and stand right in front of the cabinet to the left of the sink."
[you walk over to the cabinet]
Notice the difference. The first is "I'm here", and the second "I'm there." This is a very specific use of "I'm there", and is not equivalent to "I'm here".
"I'm here" is the general way to announce presence at any given place. By contrast, "I'm there" is typically metaphorical or figurative. As Kevin's answer says, it indicates enthusiastic assent:
"Wanna come with me to Antarctica and cuddle some penguins?"
It can also be used to mean "I understand":
"It's been so rough breaking up with her, y'know?"
"I'm there, man. Totally there with you."
Your girlfriend is correct. Here is where you are and there is someplace else. You could have been there in the past, you might go there in the future, and somebody else might be there now but in the present, wherever you are is here (from your perspective).
Google's NGram Viewer shows that "I'm here" is about 10x more common than "I'm there"
Colloquially, "I'm there!" can be used to express excitement about the prospect of going someplace, as in "Foo Fighters are playing down town tomorrow night? I'm there!"