Why does it say "ha'Ben Yakir Li Efrayim"?
Rashi: Efrayim laments, and the Shechinah says, My heart is turned to have mercy on him. He is My dear son. This is like one who has only one son - he does whatever [the son] wants.
Radak: I remember him constantly, as if he were a dear son to Me, who never sinned against Me.
Radak citing Targum Yonasan: Yisrael is dear to me, when he puts words of My Torah on his heart to do them.
Malbim: Ben Yakir is an old son. He is dear due to his attributes. If he sins, the father still loves him, for he remembers his attributes and honor. In the Midbar and in Eretz Yisrael in days of old, Yisrael were like Ben Yakir. There were Nevi'im among them, Kodesh officers and Chachamim. Then, if they sinned, He remembered the Tzadikim among them, and bore their sin. However, now, is Efrayim ha'Ben Yakir?! They have no honor and importance!
What is "Yeled Sha'ashu'im"?
Rashi: It is delightful son.
Radak: It is like a father takes delight in his son, who is dear to him.
Malbim: It is a small son. Even if he sins, the father loves him, for he is a small son in whom he takes delight. Yisrael in Egypt were like this; they did not have any attribute or perfection - "Ki Na'ar Yisrael v'Ohavehu" (Hoshe'a 11:1).
What is "Midei Dabri Bo"?
Rashi: Whenever I speak about him 1 [Radak - to Nevi'im. Also "Midei Tzeisam" (Shmuel I, 18:30) and "Midei Avro Yikach Eschem" (Yeshayah 21:2) mean 'every time.']
Rashi citing Vayikra Rabah 2:3: Dai Diburi she'Nasati Bo - it is enough that I taught to him My Torah, that I should have mercy on him.
Malbim: Whenever Dibur is followed by Beis 2 , it is for detriment.
Mesilas Yesharim (Perek 19): One who has strong love for the wife of his youth, or his only son, even speaking about them is a delight for him.
I.e. Bo, Bah, Bam, Banu, or a word starting with the prefix Beis. There are exceptions, e.g. - "Peh El Peh Adaber Bo" (Bamidbar 12:8), "v'Dibarta Bam" (Devarim 6:7)! (PF)
What is the Chidush that I remember him when I speak about him?
Radak: I remember him with the earlier love.
Malbim: All this is said in astonishment. Is Efrayim Yeled Sha'ashu'im, that even when I speak of his detriment (refer to 31:19:3:3 and the note there), I remember his attributes?!
What makes Hashem's innards yearn for him?
Radak: When He sees Efrayim in exile.
How will Hashem express His mercy on him?
Radak: He will take him out of exile.
Malbim: This is said in astonishment. Do I have mercy on him like a Yeled Sha'ashu'im?! Therefore, how can you ask Me to do wonders and love you like in days of old?!
