It depends on if they're mildly depressed, severely or trying to get better. If it's mild, then it's kind of difficult. They're depressed, so you gotta reassure them and comfort them a lot. If it's severe, it's very difficult. You have to reassure them you love them, make sure they know everythings ok, it's hard because generally they already *know* but they can't force themselves to stop being sad. It's easy but still kinda hard if they'er trying to get better. If theyre trying to get better, theyll go through lots of ups and downs, between believing in themself and not, between wanting to give up and wanting to continue fighting etc.
Either way, effort is needed, the most you can do for them is support them and love them.
Pass. I've never dated anyone with depression (by choice, I simply refuse to). I feel bad, but it makes literally everything so much harder. And it's kind of hard to enter into a partnership with someone when emotionally you HAVE to be their rock because the person on the depressive end is usually so wrapped up with their own sadness they don't even bother to try to help the other person if they're feeling down, and when they do "try" they either make it worse, or end up talking about themselves. Again.
They're very insecure, they don't realize they can hurt you, it's scary, you have to look after them and it's hard because sometimes they don't appreciate you but you have to understand what they're going through and when you try saying you understand or showing sympathy towards them they get angry and feel like you don't understand, they can be very distant, and they can hurt you as much as they hurt themselves.
It can be hard, but after time you will learn to work with them and to encourage them. I know, because I am depressed and so is my boyfriend. It can be hard at times, but after awhile you get used to how each handles certain things... :)