1. It’s raining, so we can’t have lunch in the garden.
=> If it rains, we can't have lunch in the garden.
=> Unless it rains, we can have lunch in the garden.
2. I have to work tomorrow evening, so I can’t meet you.
=> If I have to work tomorrow evening, I can't meet you.
=> Unless I have to work tomorrow evening, I can meet you.
3. I’m often tired in the morning because I often go to bed late.
=> If I go to bed late, I will be often tired in the morning.
=> Unless I go to bed late, I won't often tired in the morning.