18 The paragraph begins with the disjunctive vav, “Now as for the Lord, he.…”
19 The expression here is interesting: “he returned the captivity of Job,” a clause used elsewhere in the Bible of Israel (see e.g., Ps 126). Here it must mean “the fortunes of Job,” i.e., what he had lost. There is a good deal of literature on this; for example, see R. Borger, “Zu sub sb(i)t,” 25 (1954): 315–16; and E. Baumann, 6 (1929): 17ff.
20 This is a temporal clause, using the infinitive construct with the subject genitive suffix. By this it seems that this act of Job was also something of a prerequisite for restoration—to pray for them.
21 The construction uses the verb “and he added” with the word “repeat” (or “twice”).

In other versions

And the Lord restored the fortunes of Job, when he had prayed for his friends. And the Lord gave Job twice as much as he had before.

After Job had prayed for his friends, the Lord restored his fortunes and gave him twice as much as he had before.

After Job had prayed for his friends, the Lord restored his fortunes and doubled his previous possessions.

When Job prayed for his friends, the Lord restored his fortunes. In fact, the Lord gave him twice as much as before!

Then Yahweh returned Job’s fortune when he prayed to him on behalf of his friends. Thus Yahweh increased all that Job had twice as much as before.

And the Lord turned the captivity of Job, when he prayed for his friends: also the Lord gave Job twice as much as he had before.

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