WHETHER - IF

• To express alternatives :
  • I don't know whether/if I'll go to the party on Saturday (or not).
    
    
• To report a question :
  • Are you going to the party?
    - She asked me whether/if I was going to the party (or not).
    
    
• Only WHETHER can be used in the following cases:
  
  
  • When followed by an infinitive :
    • I don't know whether to go to the party or not.
      
      
  • After prepositions :
    • It depends on whether I get home early or not.
    • We need to talk about whether to move house.
      
      
  • When whether begins a clause which is the subject of a sentence.
    • Whether we go to the party or to the cinema makes no difference to me.
      
      
  • When followed directly by 'or not'
    • I'll be happy whether or not we go to the party.
      
      
    With whether we can use 'or not'  immediately after whether, or at the end of the sentence.
    With if we use 'or not' in end position only.
    
    
    • I called Angela to find out whether or not she was going to the party.
    • I called Angela to find out whether she was going to the party or not.
    • I called Angela to find out if she was going to the party or not.
      
      
• Only IF can be used in conditional sentences:
  • What would Max do if he lost his job?
  • I'll come to your party if you invite me.
  • If I had missed the train, I wouldn't have arrived on time.
    
    
• GENERAL RULE:
  • Use IF to introduce a condition.
  • Use WHETHER for choices and alternatives.

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