Unless

Study this example situation:

The club is for members only.

You can,t go in unless you are a member.

This means:
You can't go in except if you are a member.
You can go in only if you are a member.

Unless = except if.

• I'll see you tomorrow unless I have to work late. (= except if I have to work late)
• There are no buses to the beach. Unless you have a car, it's difficult to get there. (= except if you have a car)
• 'Shall I tell Liz what happened?'  'Not unless she asks you.' (= only if she asks you)
• Sally hates to complain. She wouldn't complain about something unless it was really bad. (= except if it was really bad)
• We can take a taxi to the restaurant - unless you'd prefer to walk. (= except if you'd prefer to walk)

• Unless we leave now, we'll be late. or If we don't leave now, we'll ...

As long as / provided / providing

• You can borrow my car {as long as, so long as} you promise not to drive too fast.
  (= you can borrow my car, but you must promise not to drive too fast - this is a condition)
• Travelling by car is convenient {provided (that), providing (that)} you have somewhere to park.
  (= but only if you have somewhere to park)
• {Providing (that), Provided (that)} the room is clean, I don't mind which hotel we stay at.
  (= the room must be clean - otherwise I don't mind)

Unless / as long as etc. for the future

• I'm not going out unless it stops raining. (not unless it will stop)
• Providing the weather is good, we're going to have a picnic tomorrow. (not providing the weather will be good)