If Clause Types

 The following explanations refer to the basic forms of 
If Clauses / Conditional Clauses

THE  FORMS  OF  IF  CLAUSES

IF CLAUSE TYPE I:

MAIN CLAUSE                                               IF CLAUSE

FUTURE                                  PRESENT / PRESENT PERFECT

ex. I will go shopping                      if the weather is fine

IF CLAUSE TYPE II:

MAIN CLAUSE                                                     IF CLAUSE

PRESENT CONDITIONAL                                 PAST TENSE                     

[would / could / should / 
might,,, + vb ]                          

ex. I would go shopping                    if the weather were / was fine

IF CLAUSE TYPE  III:

MAIN CLAUSE                                                  IF  CLAUSE                                      

PAST CONDITIONAL                                      PAST PERFECT                                                                                  

[would have /could have/

might have/should have,,, +

3^rd vb]

ex. I would have gone shopping         if the weather had been fine

Conclusions:

Type I - condition possible
Type II - condition in theory possible
Type III - condition not possible

More examples:

1. If I were you, I wouldn`t do that. - type II

Note: The form were of the verb be is usually used with all persons when speaking about If Clauses, Wish, Subjunctive,,,
Was is not wrong in these cases [ for the first and third persons, but it is informal ]

2. If she thinks she`s right, she`ll do that research on her own, anyway. - type I 

3. She would go to the seaside if she had more money. - type II

4. I would have met him if he had agreed on my plan - type III