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Why does the Torah insert the (otherwise superfluous) words "asher Yagorto Mipneihem"?
1.
Rashi: Because when Yisrael saw the unusual Makos that came upon the Egyptians, they were afraid that they should not suffer the same fate. 1
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Rashi: Which explains why the Torah wrote in Beshalach Sh'mos, 15:26 that if Yisrael will listen to the Mitzvos that Hashem commands them, they will not suffer the plagues that He brought upon the Egyptians. Since one tends to warn a person only about something of which he is afraid.