What did Hashem see?
Rashi: The Ta'amim reveal that it means that He saw that it is proper to punish Yisrael. Sending her was the punishment.
Radak: This refers to verse 10. I saw that Yehudah's Teshuvah is not of the heart; it is b'Sheker.
Malbim: I saw, recognized and showed to all that they are sent due to their adultery (idolatry).
Here it says that Hashem gave a Sefer Kerisus (Get) to Yisrael. Yeshayah (50:1) said "where is your mother's Get?!", i.e. there was no Get!
Radak: Yirmeyah discusses the 10 tribes. They received a 'Get', that they will not have a king again. When the exiles are gathered, "v'David Avdi Nasi Lahem l'Olam} (Yechezkel 37:25; a descendant of David will be king), "v'Lo Yechatzu Od li'Shtei Mamlachos" (ibid., 22). Yehudah did not get a Get; she was like a woman whose husband expelled her from the house amidst anger. He did not divorce her, for she will return to him. So the kingship will return to Yehudah. Or, we can say that [the 10 tribes received a Get] because they did not return with the exiles from Bavel, and they still did not leave the place of heir exile; it is very long. It is as if they received a Get, even though they will return. Yehudah, it is as if they did not get a Get, for they returned to their land after 70 years. Even though Yehudah was not exiled yet (when Yeshayah said that they did not get a Get), the Navi discusses the future and consoles Yisrael (Yehudah) that they will return. They were 'sold' only due to their sins. "Where is your mother's Get?!", i.e. you will return to Me soon, like you returned another time 1 . When they return, also the other Shevatim will return, for David will rule over all of them. Also Yechezkel said (37:19) "Ani Loke'ach Es Etz Yosef
This connotes that Yehudah was previously exiled, and returned. When was this?! It cannot refer to Galus Mitzrayim - we did not return soon, rather, after 210 years, which is more than the 10 tribes (Radak continues to say that they will return with Yehudah after Galus Bavel)! Perhaps it means that Hashem 'sold' (handed over) Yisrael to enemies due to their sins (Shoftim 2:14, 3:8, 4:2, 10:7) and returned to save them. Or, Yisrael will return again in Teshuvah. (PF)
This implies that they will return after Galus Bavel. In verse 12, Radak said 'at the end of days'! Refer to 3:12:3:2**.
Why does it say "v'Lo Yar'ah Bogedah Yehudah"?
Why is there a Tzeirei under the Gimel in Bogedah?
Radak: This is an exception. We find like this - "Noterah Es ha'Keramim" (Shir ha'Shirim 1:6)