I think it's conventional to say "I'm sorry" when offering condolence -- for example, "I'm sorry to hear your mum died", or, "I was sorry to hear your mum died". Although it may be conventional I have three difficulties with it: I don't know how to reply if or when it's said to me I figure it ...

I've known someone through a social activity group for about a year. At first he seemed a bit odd, but more or less harmless. He's a smart and funny guy and it can be good talking with him in short bursts (he talks a lot). But over time, he started telling me more intimate details about his life...