(Explanation in the light of Surah an-Nisa Ayah 24)
إِلاَّ مَا مَلَكْتَ أَيْمَـنُكُ Except the bondwomen you come to own: This sentence is an exception from the rule set in, the women already bound in marriage.
It means that it is not permissible that a woman who already has a husband be taken in marriage by another person, unless she comes in the ownership of a person as a bondwoman. This happened when Muslims had to carry out jihad against the infidels of Dar-al-harb (a non-Muslim state without a treaty of peace with the Muslims). As a result of a valid war with them women prisoners might have been brought to the Islamic state. If their non-Muslim husbands remained behind in their original non-Muslim state, their bond of marriage with them used to be terminated by their entry into the Islamic state, and it was lawful for a Muslim to marry her, if she was a Christian, a Jew or had embraced Islam. Similarly, if the head of the Islamic state opted to make her a bondwoman and had given her to a participant in the war as his share in the spoils, he could also enjoy her company. However, this marriage or enjoyment was permissible only after she goes through at least one menstruation period after her entry into the Islamic state. If it appeared that she was pregnant from her previous husband, it was necessary to wait until she delivers the child.
If a disbelieving woman embraces Islam in Dar-al-Harb while her husband is a disbeliever, she will be released from the bond of marriage with him after the expiry of three menstrual periods.
If a disbelieving woman embraces Islam in Dar-al-Islam and her husband is a disbeliever, then, the local legal authority should induce the husband to think about accepting Islam. If he refuses to become a Muslim, the Muslim judge should bring about a separation between the two. This separation will be taken as the divorce after which, the woman completes her period of iddah and becomes free to marry any Muslim of her choice.
Unlawfulness of Marriage with Certain Women has been Identified by Allah
كِتَـبَ اللَّهِ عَلَيْكُمْ It has been written by Allah for you: It means that the unlawfulness of marriage with women identified by Allah is something determined by Him.
All Other Women are Permissible
وَأُحِلَّ لَكُمْ مَّا وَرَاءَ ذَلِكُمْ All except them have been permitted for you: It means that women other than those who unlawfulness has been mentioned upto this point are lawful for you; for example, the daughter of an uncle, the daughter of a maternal aunt, the sister born of a maternal uncle, and the wife of a maternal or paternal uncle – after their death or after they have divorced her – subject to the condition that any other reason of prohibition does not exist. In addition to that, there is the wife of an adopted son who can be lawfully married after he divorces, or dies; so also, if one’s wife dies, he can marry her sister – these being some of the many lawful options available. All these have been covered under the generality of all except them.
It is not permissible to have more than four wives simultaneously. Detailed comments on this subject have already appeared in the beginning of Surah an-Nisa. Not finding any reference to this in the immediate context of the present ayaat should not lead anyone to misunderstand that, may be, the generality of the Qur’anic words all except them allows unrestricted marriage with women. Besides the women, marriage with whom is unlawful as declared in the Qur’an, there are several others in this category as mentioned in the ahadith, indications of which appear in the Qur’anic ayaat also and to which we have been pointing out in our comments.
(Taken from Maaruf-ul-Qur’an by Mufti Muhammad Shafi Usmani)