Tom isn’t here. He left here an hour ago.

[Alice Chu]

The first sentence is wrong because the action of leaving is completed and can’t continue anymore.
The other sentences are correct.
Is my understanding correct?

1. Tom isn’t here. He has left here for an hour.
2. Tom isn’t here. He left here an hour ago.
3. Tom isn’t here. He has just left here.
4. Tom has been leaving here for an hour, but he is still here.

 

[slevlife]

Yes, your understanding is correct, but your "correct" sentences are a bit wordy/unnatural. I'd cut the following words:

2. Tom isn’t here. He left [STRIKE]here[/STRIKE] an hour ago.
3. Tom isn’t here. He [STRIKE]has[/STRIKE] just left [STRIKE]here[/STRIKE].
4. Tom has been leaving [STRIKE]here[/STRIKE] for an hour, but he is still here.

 

[5jj]

3. Tom isn’t here. He [STRIKE]has[/STRIKE] just left [STRIKE]here[/STRIKE].

That would be he has just left for most speakers of BrE.

 

[5jj]

1. Tom isn’t here. He has left [STRIKE]here[/STRIKE] for an hour.

That is possible in some contexts, for example, he left a very short time ago, intending to return an hour after his departure.

 

[teechar]

4. Tom has been leaving here for an hour, but he is still here.

4. Tom has been leaving [STRIKE]here[/STRIKE] for an hour, but he is still here.

I'd like both Alice and slevlife to come up with a context in which that can work.

 

[Aracan]

Both seem to describe what has come to be known as "brexiting".

___________________________________________
Not a teacher

 

[slevlife]

I'd like both Alice and slevlife to come up with a context in which that can work.

Setting: A social event.
A: Has Tom left? He told me a while ago he was heading out.
B: Tom has been leaving for an hour, but he's still here. Just a moment ago I saw him saying goodbye to Bob.

 

[Glizdka]

Maybe it's just me, but I've met a lot of people who "have been quitting their job" for a long time.

I assume similar people can be found at parties, people who "have been leaving" for a few hours, but I can't be sure. I've never seen one because I'm an anti-social weirdo who never goes to parties.

 

[Tdol]

Both seem to describe what has come to be known as "brexiting".

That is a highly specialised form of leaving. ;-)