The Quran

وَإِذ تَقولُ لِلَّذي أَنعَمَ اللَّهُ عَلَيهِ وَأَنعَمتَ عَلَيهِ أَمسِك عَلَيكَ زَوجَكَ وَاتَّقِ اللَّهَ وَتُخفي في نَفسِكَ مَا اللَّهُ مُبديهِ وَتَخشَى النّاسَ وَاللَّهُ أَحَقُّ أَن تَخشاهُ ۖ فَلَمّا قَضىٰ زَيدٌ مِنها وَطَرًا زَوَّجناكَها لِكَي لا يَكونَ عَلَى المُؤمِنينَ حَرَجٌ في أَزواجِ أَدعِيائِهِم إِذا قَضَوا مِنهُنَّ وَطَرًا ۚ وَكانَ أَمرُ اللَّهِ مَفعولًا

Muhammad Asad

AND LO,¹ [O Muhammad,] thou didst say unto the one to whom God had shown favour and to whom thou hadst shown favour,² "Hold on to thy wife, and remain conscious of God!" And [thus] wouldst thou hide within thyself something that God was about to bring to light³ - for thou didst stand in awe of [what] people [might think], whereas it was God alone of whom thou shouldst have stood in awe!⁴ [But] then, when Zayd had come to the end of his union with her,⁵ We gave her to thee in marriage, so that [in future] no blame should attach to the believers for [marrying] the spouses of their adopted children when the latter have come to the end of their union with them.⁶ And [thus] God's will was done.

For this rendering of the particle idh, see surah 2, note 21. - With the above verse, the discourse returns to the problem of "elective" relationships touched upon in verses 4 ff. Several years before Muhammad's call to prophethood, his wife Khadijah made him a present of a young slave, Zayd ibn Harithah, a descendant of the North-Arabian tribe of Banu Kalb, who had been taken captive as a child in the course of one of the many tribal wars and then sold into slavery at Mecca. As soon as he became the boy's owner, Muhammad freed him, and shortly afterwards adopted him as his son; and Zayd, in his turn, was among the first to embrace Islam. Years later, impelled by the desire to break down the ancient Arabian prejudice against a slave's or even a freedman's marrying a "free-born" woman, the Prophet persuaded Zayd to marry his (Muhammad's) own cousin, Zaynab hint Jahsh, who, without his being aware of it, had been in love with Muhammad ever since her childhood. Hence, she consented to the proposed marriage with great reluctance, and only in deference to the authority of the Prophet. Since Zayd, too, was not at all keen on this alliance (being already happily married to another freed slave, Umm Ayman, the mother of his son Usamah), it was not surprising that the marriage did not bring happiness to either Zaynab or Zayd. On several occasions the latter was about to divorce his new wife who, on her part, did not make any secret of her dislike of Zayd; and each time they were persuaded by the Prophet to persevere in patience and not to separate. In the end, however, the marriage proved untenable, and Zayd divorced Zaynab in the year 5 H. Shortly afterwards the Prophet married her in order to redeem what he considered to be his moral responsibility for her past unhappiness.
I.e., Zayd ibn Harithah, whom God had caused to become one of the earliest believers, and whom the Prophet had adopted as his son.
Namely, that the marriage of Zayd and Zaynab, which had been sponsored by Muhammad himself, and on which he had so strongly insisted, was a total failure and could only end in divorce (see also next note).
Lit., "whereas God was more worthy (ahaqq) that thou shouldst stand in awe of Him". Referring to this divine reprimand (which, in itself, disproves the allegation that the Qur'an was "composed by Muhammad"), 'A'ishah is reliably quoted as having said, "Had the Apostle of God been inclined to suppress anything of what was revealed to him, he would surely have suppressed this verse" (Bukhari and Muslim).
Lit., "ended his want of [or "claim on"] her", sc., by divorcing her (Zamakhshari).
Thus, apart from the Prophet's desire to make amends for Zaynab's past unhappiness, the divine purpose in causing him to marry the former wife of his adopted son (stressed in the phrase, "We gave her to thee in marriage") was to show that - contrary to what the pagan Arabs believed - an adoptive relationship does not involve any of the marriage-restrictions which result from actual, biological parent-and-child relations (cf. note 3 on verse 4 of this surah).