What is the meaning of "Chai Chai
Rashi: When there are live people - this one is alive, and this one is alive
What is the meaning of "Av l'Vanim Yodi'a El Amitecha"?
Rashi: The father informs and directs his son's mindset to Your truth, to believe in You.
Radak: If one did not have sons, and You give to him life, and he has sons, he will inform them of Your Emes, like me. And even if he already had sons, as long as he lives with them he will teach them and inform them. If so, it is good that You extend man's life! Chazal (Brachos 10a) say that Chizkiyah had sons only after his illness. Even though the thanks and informing do not help Him - "Im Tzadakta Mah Titen Lo", "Mah Yiskan Lach" (Iyov 35:7, 3) - since He wanted and created people, He wants them to exist and go in the good path, and they will have Da'as to understand the Creator's Chasadim and thank Him for them. This is part of the good path! When their heart is proper, he helps them.
Malbim: This corresponds to "Lo Yesabru Yordei Vor El Amitecha" (verse 18). Only someone like me, that from now I will be a father to children, can inform his sons of Your fulfillment of Your promise that You promised every generation. Had I descended to the pit, Your Emes would not be fulfilled to the last generation.
Why does it say "El Amitecha"?
Radak #1: This is like Es Amitecha. Also other verses say 'El' in place of 'Es', e.g. "Asprah El Chok" (Tehilim 2:7), "va'Yirdefu El Midyan" (Shoftim 7:25). And sometimes 'Es' is in place of 'El', e.g. "v'Har'ah Es ha'Kohen" (Vayikra 13:49) and similar verses.
Radak #2: [Fathers will teach sons the path that will lead them] to Your truth. The Chesed that Hashem does with people is Emes by itself, for He is Rav Chesed; this is among His Midos. If so, it is Emes.
Radak citing Ibn Ezra, based on Targum Yonasan: Fathers will teach sons these Emunos. It is as if El has a Tzeirei 1 ; [it means these, like] "ha'Aratzos ha'El" (Bereishis 26:3).
Radak: This is astounding. In all Seforim, El has a Segol, and it is connected [via a Makaf] to Amitecha (both are read with one note. If El means 'these', it should have its own note, a 'Melech', to show that it is not read with Amitecha!), like "El Amitecha" in verse 18. The Mesorah says that these are the only two verses in which it says El Amitecha.
