39 And it came to pass at the end of two months, that she returned unto her father, who did with her according to his vow which he had vowed: and she knew no man. And it was a custom  in Israel,
39 After the two months, she returned to her father, and he did to her as he had vowed. And she was a virgin. From this comes the Israelite tradition
39 And at the end of two months, she returned to her father, who did with her according to his vow that he had made. She had never known a man, and it became a custom in Israel
39 At the end of the two months, she came back to her father. He fulfilled the vow with her that he had made. She had never slept with a man.
39 And it was so at the end of two months that she returned to her father, and he carried out his vow with her which he had vowed. She knew no man. And it became a custom in Israel
39 When she returned home, her father kept the vow he had made, and she died a virgin. So it has become a custom in Israel
Matthew Henry's Commentary on Judges 11:39
Commentary on Judges 11:29-40
(Read Judges 11:29-40)
Several important lessons are to be learned from Jephthah's vow. 1. There may be remainders of distrust and doubting, even in the hearts of true and great believers. 2. Our vows to God should not be as a purchase of the favour we desire, but to express gratitude to him. 3. We need to be very well-advised in making vows, lest we entangle ourselves. 4. What we have solemnly vowed to God, we must perform, if it be possible and lawful, though it be difficult and grievous to us. 5. It well becomes children, obediently and cheerfully to submit to their parents in the Lord. It is hard to say what Jephthah did in performance of his vow; but it is thought that he did not offer his daughter as a burnt-offering. Such a sacrifice would have been an abomination to the Lord; it is supposed she was obliged to remain unmarried, and apart from her family. Concerning this and some other such passages in the sacred history, about which learned men are divided and in doubt, we need not perplex ourselves; what is necessary to our salvation, thanks be to God, is plain enough. If the reader recollects the promise of Christ concerning the teaching of the Holy Spirit, and places himself under this heavenly Teacher, the Holy Ghost will guide to all truth in every passage, so far as it is needful to be understood.