What is the meaning of "mi'Dati Ki Kasheh Atah"?
Rashi: I know that you are hard, and if the miracle will come, and I did not tell it first, "you will say 'my idol did it" (5) - Hashem did not!' Therefore, "v'Agid Lecha me'Az b'Terem Tavo" (5). Here, the prefix Mem gives the reason - because I know... This is like "Ki me'Ahavas Hashem Eschem umi'Shamro Es ha'Shevu'ah..." (Devarim 7:8).
Radak: I already knew that you are hard - even so, I will rebuke you. This great salvation that you saw in the days of Sancheriv - it is as if you did not see it. Was there ever such hardness of heart?! So Moshe said - "Ki Anochi Yadati Es Meryecha v'Es Arpecha ha'Kasheh", "Yadati... Ki Hashches Tashchisun" (Devarim 31:27, 29).
Malbim: Now is different [than earlier miracles]. I know that it is hard for you to believe. If the miracle will come suddenly, you will not recognize that it is Elokim's finger.
What is the significance of "Gid Barzel Arpecha"?
Radak: The back of your neck is hard like an iron sinew. After you turned it to Me, you did not turn to Me again. It is as if your Oref is an iron sinew that cannot return, as if it is not of joints 1 .
Malbim: Your Oref has an iron sinew. Even if they would tell you to see the miracle, you cannot turn it back to see it.
Really, there are joints in the Oref, so a person can turn his neck. (PF)
What is the significance of "u'Mitzchacha Nechushah"?
Radak: You are not ashamed in front of Me. Arrogance is seen in the forehead and face. A brazen person lifts his forehead and face opposite the one who rebukes him, as if his forehead and face are hard; he cannot subdue them and be ashamed. Rather, he raises them brazenly. This is like "Chizku Pneihem mi'Sela" (Yirmeyah 5:3).
Malbim: Even if they show to you the miracle in front of your face, you can be brazen to deny what you saw.
