For says the LORD, see 1.8. Many translations put this at the beginning of the verse rather than after the first line.

Notice that in the text verses 1 and 2 form one continuous sentence. God is listing in verse 1 and the beginning of verse 2 what the people of Israel must do for the other nations to ask for his blessing.

This long sentence is a complicated “if … then…” structure of eight lines. It is interpreted in different ways by modern English versions, and these offer different options for translators in other languages; the two main approaches are as follows:
(1)New Revised Standard Version, Revised English Bible, and New Jerusalem Bible take the first six lines as the “if” part of the structure (that is, all of verse 1 and the first half of verse 2) and the last two lines as the “then” part. By this interpretation the last half of verse 1 and the first half of verse 2 explain what returning to the LORD (the first part of verse 1) involves. The final two lines of verse 2 are then the outcome or consequence of Israel’s genuine repentance.
(2)Good News Translation, Revised Standard Version, and New International Version take the first line as an “if” clause and the second line as a “then” clause in a separate statement. By this interpretation the sense of the first part of the Hebrew sentence is that if the people of Israel want to do something about their situation (“turn”) they should come back to (faithful worship of) the LORD. The remaining lines of the sentence then consist of an “if” part (the last half of verse 1 and the first half of verse 2), which is parallel to the “if” line at the beginning of verse 1, and a “then” part (the last part of verse 2) giving the outcome of Israel’s return to pure and faithful worship of the LORD.

Translators may follow either of the above options, using a structure that is natural for their own languages.

Return … return continues the play on words begun in 3.6. Actually, as noted above, the first two lines could have two different interpretations. The first approach sees them as a simple repetition, perhaps to emphasize the longing of God for his people to return to him. Revised English Bible, for example, has “if you will come back, if you will come back to me.” The other approach is seen in Good News Translation: “if you want to turn, then turn back to me.”

Abominations translates a noun that may be used either of pagan idols or of any other disgusting objects connected with pagan worship. Elsewhere in Jeremiah the term is used in 7.30; 13.27; 16.18 (where Revised Standard Version has “detestable idols”); 32.34. Traduction œcuménique de la Bible provides a footnote, indicating that idols are meant, while in La Bible Pléiade the noun appears as “idols.” Good News Translation translates “the idols I hate”; Luther 1984 has “your disgusting gods.”

From my presence may be taken to mean either that the Israelites had integrated and confused the worship of the LORD with that of Baal (take them “out of your worship of me”), or that the worship of Baal was being conducted in the LORD’s temple (take them “out of my temple”). However, none of the standard modern translations seems to state either view in their text. Both Moffatt (“and never stray from my sight”) and Bright (“Nor stray from my presence”) connect the phrase with the verb that follows. This is also done in one of the standard commentaries, which translates, “then there is no need for you to run from me.” However, these translations would apparently require that the conjunction and be dropped from the fourth line, for which there is no textual support.

And do not waver represents the interpretation followed by most modern translations. Good News Translation restates the meaning in a positive way: “you are faithful to me.” The verb waver is translated “wander” by Revised Standard Version in 49.30; in several other places it is rendered “bemoan,” referring to the action of shaking the head to show emotion (15.5; 16.5; 22.10; 48.17). Notice that Good News Translation reverses the order of the last two lines: “If you are faithful to me and remove the idols I hate.”

Quoted with permission from Newman, Barclay M. and Stine, Philip C. A Handbook on Jeremiah. (UBS Helps for Translators). New York: UBS, 2003. For this and other handbooks for translators see here .