Verses 18-19 repeat the idea of rejoicing (see verses 13-14). The synonymous verbs be glad and rejoice in these verses form a chiasmus.

But be glad and rejoice for ever in that which I create: God calls on his people to cast aside their current feelings of depression over the apparent failure of his promises. His new creative work will bring joy and happiness. The connector But highlights the contrast between the past that will be forgotten and the new world the LORD is making. Many versions omit this connector, but New Jerusalem Bible and Bible en français courant use “Rather,” and New Jewish Publication Society’s Tanakh has “then.” The people are to rejoice for ever, that is, permanently. The Hebrew phrase rendered for ever is literally “until until.” It means “for a very long time,” rather than “for eternity” (see the comments on 26.4).

For behold, I create Jerusalem a rejoicing, and her people a joy: For behold, I create repeats the same emphatic phrase that began the previous verse (see the comments there). Jerusalem a rejoicing, and her people a joy is an awkward English expression. It would be more natural to say “Jerusalem, a place of rejoicing, and her people, a people of joy.” Good News Translation expresses the meaning here clearly. God will restore the destroyed city of Jerusalem to be a place of celebration and joy for its inhabitants (compare 51.11).

Bible en français courant expresses vividly that there will be non-stop joy in Jerusalem by rendering this verse as “Rather, rejoice and don’t stop voicing your enthusiasm loudly for what I will create: an enthusiastic Jerusalem with its people overflowing with joy.” Other possible models are:

• But be happy and joyful forever
because of what I am creating.
Look, I am creating a happy Jerusalem
and joy for all its people.

• But now rejoice and be glad always
because of what I am about to create.
Look, I will create a Jerusalem full of happiness,
with all its people shouting for joy.

Quoted with permission from Ogden, Graham S. and Sterk, Jan. A Handbook on Isaiah. (UBS Helps for Translators). New York: UBS, 2011. For this and other handbooks for translators see here .