If We’ll have the party in the backyard if the weather’s good. If anyone calls for me, just say I’ll be back at 4 o’clock. Would you mind if I opened the window (= May I open it)?
If = when If I don’t have a cup of coffee in the morning, I’m useless.
If = whether Mrs. Kramer called to ask if her cake was ready.
Unless ( If not ) She won’t go unless you go (= If you do not go, she will not go either, but if you go, she will go). You can’t get a job unless you’ve got the experience (= You can only get a job if you have experience).