1 You can't visit the United State unless you get the visa.
-> If you got a visa, you could visit the United States.

2 Unless someone has a key, we can't get into the house.
-> We can only get into the house if someone has a key.

3 You will catch a cold if you don't keep your feet dry.
-> Unless you keep your feet dry, You will catch a cold

4 Joan will stay on at school unless she finds a good job before September.
-> If she finds a good job before September ,Joan won't stay on at school.

5 If it doesn't rain soon, millions of pound's worth of crops will be lost.
-> Unless it rain soon,millions of pound's worth of crops will be lost.