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 :The modal fallacy is already explained in the article. Formally stated, the modal fallacy is as follows: 
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 :1. It is necessary that if A is true, then B is true. 
 :2. A is true. 
 :3. Therefore, it is necessary that B is true. 
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 :The modal fallacy occurs when someone takes a necessary truth (if A is true, then B is true), and then applies that necessity to one of the individual parts (B is true). Here is an example that illustrates this: 
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 :1. If Dave has two sons, then Dave has to have at least one son. 
 :2. Dave has two sons. 
 :3. Therefore, Dave has to have at least one son. (It is necessary that Dave has at least one son.) 
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 :This proof is incorrect, since it is not logically necessary that Dave has at least one son. He could have chosen to have no children, or he could have had all daughters. Just because something is true, that does not make it necessarily true. More properly stated, the proof goes like this: 
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 :1. It is necessary that (if Dave has two sons, then Dave has at least one son). 
 :2. Dave has two sons. 
 :3. Therefore, Dave has at least one son. 
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 :When the proof is phrased properly, and the modal fallacy does not occur, then the statement (Dave has at least one son) is shown to be true, but not necessarily true. So if you apply this to the argument from free will, the proof is as follows: 
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 :1. It is necessary that (if God knows that a person will do action A at time T, then that person will do action A at time T). 
 :2. God knows that a person will do action A at time T. 
 :3. Therefore, the person will do action A at time T. 
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 :The modal fallacy would be changing the last statement to, “Therefore, it is necessary that the person will do action A at time T.” Concluding that someone will do something is very different from concluding that they must do something. I really don’t know how to explain it any better.