In class today I was using a Martin Luther King biography to teach the passive. Here's my question: several of the students got stumped on "Martin Luther King Jr. was father to four." They didn't understand why "to" wasn't followed by a verb and I couldn't explain it to them. I know we use other expressions like "party to s.t." and "witness to s.t" etc.
Does anyone have a clear, simple explanation? I've browsed through the forums, but nothing that might be an answer caught my eye.