If he finds it can also be translated here as “When he finds it,” if that is better in the receptor language.

Truly, I say to you (first introduced in 6.2) translates the same formula used in verse 3. Here it serves to emphasize the words of Jesus which follow.

He rejoices over it more than over the ninety-nine that never went astray: for readers who already understand the intention of the parable, this statement will make sense, but for others this will not be the case, for it will not seem logical. Ninety-nine sheep are certainly worth more than any one sheep, and no sensible shepherd would leave a large number of sheep at the mercy of predators in order to look for one lost sheep. In the interpretation and translation of these words, two things should be kept in mind: (1) the parable represents an intended exaggeration, introduced for the sake of emphasis; and (2) the emphasis in the parable is upon a specific event, the recovery of a lost sheep, in contrast to the usual circumstances in which all the sheep remain safely within the fold. The meaning may then be stated “A shepherd who has a hundred sheep is happy when ninety-nine of them do not get lost. But what really makes him happy is when he finds the one sheep that did get lost.” Or “A shepherd is happy when ninety-nine of his sheep do not wander away and get lost. But what really makes him happy is when he finds the one that did get lost.” Another possibility is “The joy that man has because he found that one sheep is much more than the joy he had because of the other ninety-nine who didn’t get lost.”

Quoted with permission from Newman, Barclay M. and Stine, Philip C. A Handbook on the Gospel of Matthew. (UBS Handbook Series). New York: UBS, 1988. For this and other handbooks for translators see here .