Ask Annie: My housemate’s partner doesn’t contribute to rent. How do I broach the subject?

Dear Annie, My housemate’s partner has moved in and he doesn’t contribute to rent or bills. How do I broach the subject? This seems to be a common enough thing that happens but for some reason I’m struggling with it. The obvious answer would be to just ask him to pay rent, but I’ve heard of many instances where it ends in a big old falling out. I could really do with some advice as...