After quitting drinking, he was a completely different person from before.
Tips
history
From the late-Qing novel 《文明小史》 by Li Boyuan (李宝嘉): a magistrate who used to be flexible and accommodating now acts the opposite — 何以如今判若两人? 'How is it that now he seems like a different person entirely?'
usage
Often paired with 跟/与/和...相比 (compared to...) or 与过去 (compared to before) to mark the contrast. Can describe positive change or worrying transformation.
