1.

When the LORD turned again the captivity of Zion, we were like them that dream.

1-3. When the Lord, c.—The joy of those returned from Babylon was ecstatic, and elicited the admiration even of the heathen, as illustrating God's great power and goodness.
turned again the captivity—that is, restored from it (Job 39:12 Psalms 14:7; Proverbs 12:14). HENGSTENBERG translates: "When the Lord turned Himself to the turning of Zion" (see Margin), God returns to His people when they return to Him (Deuteronomy 30:2; Deuteronomy 30:3).

2.

Then was our mouth filled with laughter, and our tongue with singing: then said they among the heathen, The LORD hath done great things for them.

3.

The LORD hath done great things for us; whereof we are glad.

4.

Turn again our captivity, O LORD, as the streams in the south.

4. All did not return at once; hence the prayer for repeated favors.
as the streams in the south—or, the torrents in the desert south of Judea, dependent on rain (Joshua 15:9), reappearing after dry seasons (compare Job 6:15; Psalms 68:9). The point of comparison is joy at the reappearing of what has been so painfully missed.

5.

They that sow in tears shall reap in joy.

5, 6. As in husbandry the sower may cast his seed in a dry and parched soil with desponding fears, so those shall reap abundant fruit who toil in tears with the prayer of faith. (Compare the history, Ezra 6:16; Ezra 6:22).

6.

He that goeth forth and weepeth, bearing precious seed, shall doubtless come again with rejoicing, bringing his sheaves with him.

6. He that goeth forth—literally, better, "He goes—he comes, he comes," c. The repetition implies there is no end of weeping here, as there shall be no end of joy hereafter (Isaiah 35:10).
precious seed—rather, seed to be drawn from the seed box for sowing literally, "seed-draught." Compare on this Psalm, Isaiah 35:10, &c.