they’re / their / there
Three words. Three jobs. You are picking one at random.
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Wrong: Their going to be late.
Right: They’re going to be late.
Expand it: “they are going to be late.” If that works, it’s they’re.
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Wrong: The house lost they’re roof.
Right: The house lost their roof.
The roof belongs to them. Possession is their.
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Wrong: Put it over their.
Right: Put it over there.
A place. It has here inside it, which is also a place.
The test
Say “they are” out loud in place of the word. If the sentence survives, it’s they’re. If it doesn’t, ask whether the thing belongs to someone—that’s their. Everything left over is there.
That’s it. There is no fourth case, and no situation where you get to guess.
Why nobody catches it
They sound identical, so your ear is no help and spellcheck stays quiet—every one of them is a real word, correctly spelled, just doing someone else’s job. The only fix is knowing the rule, which now you do.
Now go reread the email you just sent.