What's the best way to respond to hearing about someone's past abuse?

My instinct is to just listen. Absorb. And be there for them.

I don't want to come off as I'm not really listening, but honestly it doesn't feel appropriate to say any of the generic responses many often respond with when they hear about someone's past suffering.

I don't even know if I should say "I'm sorry." Sometimes that's the only thing I want to say though, if anything.