If he finds it ... that did not go astray

This is the end of the parable that begins with the words "If anyone" in verse 12. (See: rc://en/ta/man/translate/figs-parables)

truly I say to you

"I tell you the truth." This adds emphasis to what Jesus says next. The word "you" is plural. (See: rc://en/ta/man/translate/figs-you)