Why did he say "Lachen"?
Rashi: Because I suffer these afflictions.
Malbim: Because Yirmeyah requested [constant] Nevu'ah and that his Tefilah for rain be answered, Hashem answered this.
To whom did Hashem say so?
Rashi: To Yirmeyah.
What is the meaning of "if you will return, I will return you; you will stand in front of me"?
Rashi, Mahari Kara: Yirmeyah told Yisrael, [Hashem told me] if you will return, Hashem will return me to stand in front of Him.
Radak: Yirmeyah said improperly about Me (you cannot rely on Me). If you will repent and regret what you said, I will pardon you and return you to stand in front of Me, like before. Standing in front of Hashem is Tefilah, thanking Him and receiving Nevu'ah, like "if Moshe and Shmuel would stand in front of Me" (verse 1), "v'Avraham Odenu Omed Lifnei Hashem" (Bereishis 18:22). It says so only about a Navi.
Malbim: If you will get the nation to repent, I will return you to constant Nevu'ah; you will always stand in front of Me.
What is the meaning of "Totzi Yakar mi'Zolel"?
Rashi: Take out a proper person from a Rasha, i.e. to cause him to repent.
Radak: If you will toil and make the Resha'im repent.
Malbim: [Even if you will not get the entire nation to repent, if at least you will return the dear Nefashos, and separate them from the Resha'im.
How will he resemble Hashem's mouth?
Rashi: I make a decree, and you nullify it.
Radak #1, Malbim: What you decree and say will be fulfilled as if I say it. This is like it says about Shmuel ha'Navi "v'Lo Hipil mi'Kol Davarav Artzah" (Shmuel I, 3:19).
Radak citing Targum Yonasan: You will fulfill the desire of My words.
Radak #2: You will be like I said, "a fortified city, [iron pillar] and copper wall" (1:18).
What is the meaning of "they will return to you, and you will not return to them"?
Rashi citing Targum Yonasan: They will return to your words, and you will not return to err after (follow) them. Radak - be careful in your attempt to make them repent, lest they make you err via their many claims.
Malbim: Yirmeyah had complained that the Resha'im pursue him. Hashem said, do not worry; they will not force you to agree to their actions and not prophesize to them. Also refer to 15:20:2:2.
