Zero Conditional

When we want to talk about things that are always or generally true, we can use

If/When/Unless plus a present form PLUS present simple or imperative.

• 
  If you press this button, you get black coffee.
  
• 
  When you fly budget airline, you don't expect to get anything to eat.
  
• 
  Unless you need a lot of leg-room, don't pay the extra for first class.
  

Notice that we are talking about something which is generally true, not a specific event.

In the condition clause,
there can be a variety of present forms. In the result clause, there can
only be the present simple or imperative.

• 
  If you visit Barcelona, look out for the spectacular architecture.
  
• 
  If unemployment is rising, people tend to stay in their present jobs.
  
• 
  If you've finished everything, go home.
  
• 
  When you go to Barbados, take plenty of sun cream.
  
• 
  When I'm working, please be quiet.
  
• 
  When I've written a new article, I run it through my spell-checker.
  

Notice that 'unless' means the same as 'if not'.

• 
  Unless he asks you to continue, stop all work on the project.
  
• 
  Unless interest rates are rising, it's not a good investment.
  
• Unless you've been to Tokyo yourself, you don't really understand how fantastic it is.