Deuteronomy 21:16

Or, while the son of the hated liveth.

Geneva Bible (1599) marginal notes — public domain. Text via Bolls.life.

16. in the day that he causeth his sons to inherit] When he makes his will, Gen_24:36; Gen_25:5; cp. 2Sa_17:23, 2Ki_20:1.

before] in preference to (see on Deu_5:7), R.V. margin is improbable.

The Cambridge Bible for Schools and Colleges (gen. ed. J. J. S. Perowne; Cambridge University Press, 1878–1918) — public domain. Text from a community e-Sword module.

Then it shall be, when he maketh his sons to inherit that which he hath,.... By a will in writing, or byword of mouth, or by a deed of gift, actually bestowing his goods upon them, and dividing among them what he is for the present possessed of; see Luk 15:12,

that he may not make the son of the beloved firstborn before the son of the hated, which is indeed the firstborn; that is, when such is the case, that the son of his wife he has the least value for is really his firstborn, he may not, through favour and affection to the wife he loves better, prefer her son, and declare him to be the firstborn, by devising to him or bestowing on him the double portion of his goods; for so to do would not be right, or agreeably to the will and law of God; for though previous to this law the birthright was given to Joseph, the eldest son of Rachel, the most beloved wife of Jacob, before Reuben who was the son of Leah, less beloved by him, and was in fact his firstborn; yet this was owing to the sin of Reuben, and by the appointment of God; see Gen 49:3.

John Gill's Exposition of the Whole Bible (1746–1763) — public domain (CC PDM 1.0). Text via the HelloAO Bible API.

Hated, or less loved. (Haydock) --- The inheritance goes to the first-born independently of the father’s disposition, in order to prevent the disturbances which would otherwise have taken place in families, where the different wives would have been continually endeavouring to get their respective children preferred before the rest. David, indeed, assigned the throne to Solomon, to the exclusion of Adonias; but this was done by the command of God, 3 Kings i. 17. The regulation of Jacob, in favour of Joseph, was made prior to this law. The Jewish doctors inform us that a father cannot disinherit any of his lawful heirs, except the judges ratify his sentence, while he is in health. But if he be dangerously ill, his verbal declaration will suffice, provided he appoint some one whom the law does not reject. For if he were to make a Gentile his heir, the will would be null. The testament must be made in the day time, for which they cite Ecclesiasticus xxxiii. 24. They say likewise that a father may, while living, give his effects to whom he pleases, and by this means disinherit his children; or he may give the succession to one of them, who is then considered as a tutor of the rest, and is bound to maintain them with necessaries till the year of jubilee, when each may claim his respective share. (Selden, Succes. c. xxiv.) But all these regulations seem to contradict the law. (Calmet)

George Leo Haydock (1774–1849), Haydock's Catholic Family Bible and Commentary, 1859 edition — public domain. Excludes Psalms (Douay-Rheims chapter numbering differs from Protestant) and Deuterocanonical books (Catholic canon only).

He may not; it is not lawful, because contrary to the rights and law of nature.

Before the son, or, before the face of the son, i.e. in his lifetime, as this phrase is understood, Gen 11:28; 16:12; 25:18. And when this phrase is rendered before another, it signifies only in the presence of another, but never notes the preference of one person to another, which the Hebrews express in another manner. And this may be added to intimate, that if the eldest son were dead, and had left a child, the father was free to give the right of his first-born unto his second son, rather than to the child of the eldest. Or this phrase may be an aggravation of the fact, whereby his father did in a manner spit in his face, and fasten a reproach upon him in his very sight and presence.

Matthew Poole's English Annotations on the Holy Bible (1683–1685) — public domain. Text from a community e-Sword/MySword module.

He may not make; literally, is not able to make; i.e. is legally incapable of making.

The Pulpit Commentary (1880–1919), ed. H.D.M. Spence-Jones & Joseph S. Exell — public domain. Text from a community e-Sword module.

LO YUCHAL’ [literally: “he could not” but rendered: HE MAY NOT] DECLARE THE SON OF THE BELOVED FIRSTBORN. This is an admonition against doing so. Likewise are all such verses as: ‘Lo thuchal’ (thou mayest not) eat within thy gates etc.; ‘lo thuchal’ (thou mayest not) put a foreigner over thee., 17:15. And in all these cases, Onkelos translates “you have no right” [which proves that the verse before us, too, constitutes an admonition], and the purport of these expressions is, “you could not allow yourself the possibility of doing so,” emphasizing the importance of the admonitions. Similarly, But he shall acknowledge the firstborn, the son of the hated, by giving him a double portion constitutes a positive commandment that he bequeath a double portion to the firstborn. Thus he who equalizes the firstborn with his brothers [i.e., he assigns him a share equal to theirs] violates both a negative commandment and a positive commandment, even though his words are not legally valid. He surely violates both [commandments] if, knowing he [the son of the hated one] is the firstborn, and did not wish to make it known that he is the firstborn and claimed that he is an ordinary son and caused him to inherit like any one of his sons. Now, these are newly-declared commandments. And from that which Scripture states [he may not declare the son of the beloved firstborn] in the face of the firstborn, the son of the hated it would appear to me that this commandment and this law are binding only during the lifetime of the firstborn. But, if the firstborn died during the life of his father, even though he [the deceased son] inherits his share as the firstborn in the grave and bequeathes it by law to his children, yet if the father wished [to deviate] and said, “My sons shall inherit such-and-such of my belongings, and the children of my [late] son, the firstborn, shall take such-and-such of my belongings,” his instructions are valid just as they would be valid in a case where there is no firstborn son. Similarly, the father would not be violating this negative commandment if he did not acknowledge the firstborn son only after his death, for I have never found the expression ‘al pnei’ (in the face of) except with reference to the living, such as: ‘al pnei’ (in the presence of) Aaron their father; ‘al pnei’ (in the presence of) his father Terah, and similarly all such expressions. [Therefore, in this case, since it states that the father may not do it ‘al pnei’ the firstborn of the hated, it follows that the prohibition is binding only during the lifetime of the firstborn and that the father had acknowledged him as such.].

Ramban (Nachmanides) on the Tanakh — English translation: “Commentary on the Torah by Ramban (Nachmanides). Translated and annotated by Charles B. Chavel. New York, Shilo Pub. House, 1971-1976” (CC BY 4.0). Text via Sefaria.

He may not prefer the son of the loved one. The birthright of the eldest son may not be transferred due to love or hatred for one or another of his wives. However, if the father transfers the birthright due to the wicked behavior of the son, then he acts appropriately. As our Sages have said: If the son does not act in a proper manner, a father who disinherits him will be remembered for good (Bava Basra 133b). This is what Yaakov did, as the verse states, “Because he defiled his father's couch, his birthright was given to the sons of Joseph the son of Yisroel” (I Divrei HaYomim 5:1).

Sforno on the Tanakh — English translation: “Yalkut, Sifsei Chachomim Chumash, Metsudah Publications, 2009” (CC BY 4.0). Text via Sefaria.