“Oh! You trained him to stand on his hind legs?”
While walking around your neighborhood, you see your neighbor, who's playing with her dog. The dog is standing up like a person. You're delighted to see that, so you say this.
Oh! You trained him to stand on his hind legs?
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Oh
This is a sound that people make when they've learned something that surprises them, or something that they didn't know before.
Oh, you're not coming? I thought you were.
A: Excuse me, miss. We're closing in five minutes.
B: Oh, OK.
train (an animal) to (do something)
You "teach" people, but you "train" an animal.
You can sometimes use the word "train" for a person, but it suggests training for fighting or the military. And it's also common to joke about women "training" their husbands or boyfriends:
I had to train him not to leave the toilet seat up.
(an animal's) hind legs
Animals that walk on four legs, like dogs, have "front legs" and "hind legs".