CONDITIONALS WITH UNLESS

UNLESS means the same as if...not (=A NO SER QUE). Like if, unless is followed by a present tense, a past tense, or a past perfect tense (never by a conditional). Unless is used instead of if...not in conditional sentences of all types. 

The order of the clauses doesn't matter with sentences using unless.

Unless you object, we'll bring some champagne.

Unless is often used in conditional sentences. It can be used with the first, second, or third conditional. Here are some examples.

First Conditional: "You'll be unhappy unless you break up with her" = "You'll be unhappy if you do not break up with her."

We will arrive at 8:00 if our train is on time. 

Second Conditional: "I wouldn't ask her out unless you told me it was OK" = "I wouldn't ask her out if you didn't tell me it was OK."

Third Conditional: "They wouldn't have come over unless we'd invited them" = "They wouldn't have come over if we had not invited them."

Unless is good to use when it highlights an exception to what is generally true. In this example I only have one reason why i may be late: traffic (vehicles on the road):

"I'll arrive at 10am unless there is traffic."
"I'll arrive at 10am if there isn't any traffic."

You can practise this HERE