CONDITIONALS

   first conditional

   This refers to a grammar structure used to talk about the ‘likely’ result of something
   happening or not happening. The grammar structure is:

   if-clause: 'if' + present tense (eats)

   main clause: will or won't

   If he takes these antibiotics, he’ll get better quickly. If he doesn’t, he won’t.

   Note! Certain other modal verbs (for example, might, but not would), can be used in the main
   clause.

   second conditional

   This refers to a grammar structure used to talk about an ‘unreal’ or ‘unlikely’ situation. The
   grammar structure is:

   If-clause: 'if' + past tense (ate)

   Main clause: 'would' or 'wouldn't'

   If my shares went up 500% I’d sell them instantly. (but it’s unlikely that they will go up
   500%)

   Note! Could and might can also be used in the main clause.

   third conditional

   This refers to a grammar structure used to imagine the impossible. It is impossible because
   something happened in the past and can’t be changed. The grammar structure is:

   If-clause: 'if' + past perfect (had eaten)

   Main clause: would have + past participle (eaten)

   You would have passed your exam if you had studied harder. (but you didn’t study hard and you
   didn't pass your exam)

   Note! Could have and might have can also be used in the main clause.

   

   

   

   Another source:

   http://www.usingenglish.com/articles/english-conditionals-an-introduction.html

   

   

   MIXED CONDITIONAL SENTENCES

   It is possible for the two parts of a conditional sentence to refer to different times, and
   the resulting sentence is a "mixed conditional" sentence. There are two types of mixed
   conditional sentence:

   A. Present result of past condition:

   1. Form
   The tense in the 'if' clause is the past perfect, and the tense in the main clause is the
   present conditional:

   'IF' CLAUSE                              MAIN CLAUSE                            
   If + past perfect                        Present conditional                    
   If I had worked harder at school         I would have a better job now.         
   If we had looked at the map              we wouldn't be lost.                   

   

   2. Function
   In these sentences, the time is past in the 'if' clause, and present  in the main clause. They
   refer to an unreal past condition and its probable result in the present. They express a
   situation which is contrary to reality both in the past and in the present:
   'If I had worked harder at school' is contrary to past fact - I didn't work hard at school,
   and 'I would have a better job now' is contrary to present fact - I haven't got a good job.
   If we had looked at the map (we didn't), we wouldn't be lost (we are lost).

   Examples:

   ·        I would be a millionaire now if I had taken that job.

   ·        If you'd caught that plane you'd be dead now.

   ·        If you hadn't spent  all your money on CDs, you wouldn't be broke.

   B. Past result of present or continuing condition.

   1. Form
   The tense in the If-clause is the simple past, and the tense in the main clause is the perfect
   conditional:

   'IF' CLAUSE                          MAIN CLAUSE                                
   If + simple past                     Perfect conditional                        
   If I wasn't afraid of spiders        I would have picked it up.                 
   If we didn't trust him               we would have sacked him months ago.       



   

   

   2. Function
   In these sentences the time in the If-clause is now or always, and the time in the main clause
   is before now. They refer to an unreal present situation and its probable (but unreal) past
   result:

     o 'If I wasn't afraid of spiders' is contrary to present reality - I am afraid of spiders,
       and 'I would have picked it up' is contrary to past reality - I didn't pick it up.
     o 'If we didn't trust him' is contrary to present reality - we do trust him, and 'we would
       have sacked him' is contrary to past reality - we haven't sacked him.

   Examples:

   a.     If she wasn't afraid of flying she wouldn't have travelled by boat.
   b. I'd have been able to translate the letter if my Italian was better.
   c. If I was a good cook, I'd have invited them to lunch.
   d. If the elephant wasn't in love with the mouse, she'd have trodden on him by now.