Solution : Unless, Provided/Providing/On condition that

• If you don't work hard today, you can't have a holiday tomorrow.
  Unless you work hard today, you can't have a holiday tomorrow.
  

• I'll lend you the book if you return it next Monday.
  I'll lend you the book provided (that) you return it next Monday.
  

• I won't lend Peter money if he doesn't need it.
  I won't lend Peter money unless he needs it.
  

• If I don't have a quiet room, I won't able to do any work.
  Unless I have a quiet room, I won't be able to do any work.
  

• Go on to the next unit if you have already finished Unit 2.
  Go on to the next unit providing (that) you have already finished Unit 2.
  

• You can camp in my field if you promise to leave no mess.
  You can camp in my field on condition that you promise to leave no mess.

• Unless you wear boots, you may get bitten by snakes.
• If he goes into that field, the bull will chase him.
• You can't make tea unless you have hot water.
• He will divorce his wife unless she confesses her sins.
• Someone will steal your car if you leave it unlocked.