If her father says OK, John will marry Mary.    

Now what you see is a Main Clause and the IF Clause, and they make a Conditional sentence.  In this case the Main Clause is Future Simple (S + WILL/WILL NOT + V1) and the IF Clause was Present Simple (S + V1).  Either clause can go first. (John will marry Mary if her father says OK.)

Here is a Negative: (IF Clause) If her father DOESN'T say OK ,  (Main Clause) John will NOT marry Mary.  (IF + S + DOES NOT + V1 + [then] S + WILL NOT + V1)

 

WHY: To use the Conditional Mood we must first ask WHY we are using it. (For what purpose) What Degree of Probability do we want to show?
1. PROBABLY (90%)- If Mary's father says OK, John will marry her.
2. POSSIBLY (50%)   - If I studied, I would pass the test.
3. TOO LATE   - If I had studied, I would have passed the test.

HOW: (You must answer WHY first, to know the Degree of Probability.  Then you choose the supporting verb tenses. In our first case:

If Mary's father says OK, John will marry her.  (It Probably will happen.)

...we were looking from Present (If Clause) to >>>>Future (Main Clause).  The direction in the Conditional is ALWAYS from an earlier point in time to a later one. Verb tenses must show that time element.

Now you try this sentence with the second WHY, a POSSIBLY sentence.
(IF + S + V2 + [then] S + WOULD + V1)
would car lot I a drive if bought
____ ____ ____ ____ ____ ____ _____ ____ ____ ____.


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