Overheard ... in Dundrum

A café at the Dundrum Town Centre. Two au pairs are talking. Well, one of them is talking, with a German accent, while the other, who looks vaguely Slavic, listens in sympathetic silence. The German is talking about relationship problems. Party Soldier, who is sitting at the next table, cannot help overhearing.

"It's a very hard situation," the girl says, "I don't know what happens when I'm coming home."

Party Soldier bristles a little at this. What are they teaching them at schools these days? He would like to correct her:

"Excuse me, it's future tense."

"What?"

"You say I don't know what will happen when I go home, not I don't know what happens when I'm coming home. It's the future simple tense. Why are you using the present tense to talk about the future?"

"Oh, okay - but isn't when I'm coming home in the present tense?"

"Well, yes, but that's different."

"Why?"

"Because when always takes the present, even when referring to the future ... and anyway, it's when I come, not when I'm coming.

"Why?"

"Because the continuous only refers to something going on over a period of time, and your coming home is a single action. You could say when we're shopping tomorrow, but it's when I come home."

"But it takes a long period of time, several hours. Longer than shopping."

"Look, never mind. And anyway it's when I go home, not when I come.

"Why?"

"Because - because it just is, all right?"

Who would have thought that so much tedious grammatical correction could be gleaned from such a short sentence?

I hope it works out for her anyway.

Now why is that in the present tense ...