What is the difference between Shemi'ah, Asiyah and Ha'azanah?
Rashi: Shemi'ah means acceptance, Asitah, performing and Ha'azanah, to listen and observe with great care.
Ramban #1 (citing Ibn Ezra): "Shamo'a Tishma" means to know the reasons for the Mitzvos, "ve'ha'Yashar be'Einav Ta'aseh" refers to the Mitzvos Asei and "ve'Ha'azanta le'Mitzvosav" to the Mitzvos Lo Sa'aseh.
Ramban #3 (citing the Mechilta): "ve'ha'Yashar be'Einav Ta'aseh" refers to doing business with integrity. 1
Because someone who does business with integrity and is liked by the people, it is as if he kept the entire Torah (Ramban citing the Mechilta).
Why did Hashem mention the plagues of Egypt here?
Rashi (in Ki Savo, Devarim 28:60): When Yisrael saw the unconventional plagues which struck Egypt, they were frightened that they would not suffer the same fate; 1 therefore Hashem assured them (here) that they had nothing to fear as long as they kept the Mitzvos, but warned them (there) that they would, if they did not. 2
Ramban (citing Ibn Ezra): Following the downfall of the Egyptians, this was the first miracle that took place in the desert, and was the reverse of the first of the ten plagues - since there Hashem made the sweet water bitter, whereas here, He made the bitter water sweet, It sent a message to Yisrael, that He has the ability to perform two opposite miracles, to encourage them to fear Him and not to turn against Him, and to love Him, since He would do good to them just as He had healed the water on their behalf.
Rashbam: Hashem tested Yisrael by depriving them of water and subsequently healing it, He was showing them that, if they would adhere to the Mitzvos that He was teaching them, He would not plague 3 them in the way that He had plagued Egypt by turning their water into blood.
Rabeinu Bachye points out that the Gematriyah of the word asher ("Kol ha'Machalah asher Samti be'Mitzrayim Lo Asim alecha" ) is equivalent to that of the first letters of the ten plagues that were engraved on Moshe's stick ('D'tzach, Adash, be'Achav').
Since one tends to threaten a person with something that he is afraid of (Rashi).
If Hashem will not place the plagues of Egypt upon Yisrael, then what does "Ki Ani Hashem Rof'echa" mean?
Rashi #1: Hashem promises that He will not strike us with the plagues of Egypt and that if and when He does, it is as if He had not - because whereas the plagues destroyed the Egyptians, He strikes Yisrael like a doctor operates on a patient - for his good, to enable him to survive and continue living. 1
Rashi #2: Because, like a good doctor, teaches Yisrael how to avoid the plagues of Egypt 2 - by performing the Mitzvos that He is teaching them.
Ramban: It means that in the event that Yisrael are stricken with natural illnesses (Refer to 15:26:2:3*), He is their doctor and will cure them.
Targum Yonasan: Hashem will not strike Yisrael with the plagues of Egypt, if they listen to Hashem ... ; but if they don't, 3 and He strikes them, then, if they do Teshuvah, He is their doctor, and He will cure them.