B"H

QUESTION:
How do we know (by looking at one of the 10 commandments in the Book of Shemos) that if a man does wrongly, he is punished four-fold; but if he does good, he is rewarded 500-fold?

ANSWER:
The Torah says there (Shemos 20:5): "...for I am the L-ord thy G-d, even a jealous G-d, who remembers the iniquity of the fathers upon their children for three and for four generations of those that hate me. Yet, do I show mercy upon thousands that love me and keep my commandments."

The Rabbis, especially Onkelos in his Aramaic translation of this verse, say that the second clause of this passage which speaks about "thousands that love me" is actually congruous with the first clause. Just as the first clause speaks about "generations," so does the second clause speak about "generations." So the sense would be "thousands of generations that love me." Since the word "generations" is written in the plural form, this would also imply that the least possible number of anything which is made plural is TWO. The sense is now then, "Yet, do I show mercy upon two thousand generations that love me, etc."

So, here, is its expounded meaning: G-d punishes a man up until the fourth generation, but he rewards a man up until the two-thousandth generation. The ratio between G-d's punishing a man for iniquitous living and of G-d's rewarding a man who serves and loves Him is the ratio between 1/500.

Sincerely,
David

B"H
The question should rather have been:

"How do we know (by looking at one of the 10 commandments in the Book of Shemos) that if a man does wrongly, he is punished four-fold; but if he does good, he is rewarded 2,000-fold?"