A.T. Robertson Commentary Luke 15:4

"What man of you, having a hundred sheep, and having lost one of them, doth not leave the ninety and nine in the wilderness, and go after that which is lost, until he find it?" — Luke 15:4 (ASV)

In the wilderness (εν τη ερημω). Their usual pasturage, not a place of danger or peril. It is the owner of the hundred sheep who cares so much for the one that is lost. He knows each one of the sheep and loves each one.

Go after that which is lost (πορευετα επ το απολωλος). The one lost sheep (απολωλος, second perfect active participle of απολλυμ, to destroy, but intransitive, to be lost). There is nothing more helpless than a lost sheep except a lost sinner. The sheep went off by its own ignorance and folly. The use of επ for the goal occurs also in Mt 22:9; Acts 8:26; 9:11.

Until he find it (εως ευρη αυτο). Second aorist active subjunctive of ευρισκω, common verb, with εως, common Greek idiom. He keeps on going (πορευετα, linear present middle indicative) until success comes (effective aorist, ευρη).