Anyone seeking to commentate on the Qur'an should at least be familiar with the following fifteen subjects:
01) Lughat, i.e, philology of language, which helps in understanding the appropriate meanings of words. Mujahid (ra) says: "one who believes in Allah and the Day of Judgement should not open his lips in respect of the Qur'an, unless he is thoroughly conversant with the philology of the Arabic language. Quite often an Arabic word has several meanings. A person may be knowing only one or two of them, though in a given context the actual meaning may be quite different."
02) Nahw, i.e, syntax, a branch of grammar, which helps in understanding the relation of a sentence with another and also of I'raab (vowel sounds) of the letters of a word. A change in I'raab often means a change in meaning.
03) Sarf, i.e, etymology, a branch of grammar, which helps in knowing the root words and conjugations. The meaning of a word changes with the change in the root and a change in its conjugation.
04) Ishtiqaaq, i.e, derivatives. It is necessary to have the knowledge of derivatives and their root words, because if a word has been derived from two different root words, it will have two different meanings, e.g., the word 'maseeh' is derivable from 'masah' which means to touch or to move wet hands over, and also from 'masaahah' which means measurement.
05) Ilmul Ma'aani, i.e, knowledge of semantics, because phrase constructions are understood from their meanings.
06) Ilmul Bayaan, i.e, knowledge of figures of speech, like similes and metaphors, due to which expressions or shades of meaning or similes and metaphors become known.
07) Ilmul Badee', i.e, knowledge of rhetoric, the knowledge which reveals the beauty of language and its implications.
08) Ilmul Qiraa'ah, i.e, knowledge of the art of pronunciation, because different methods of recitation sometimes convey different meanings, and sometimes one meaning is to be preferred over the other.
09) Ilmul Aqaa'id, i.e, knowledge of the fundamentals of faith. This is necessary to explain certain analogies. The literal meaning of certain ayaat referring to Allah is not the correct one. For example, the analogy in the ayat- (The hand of Allah is over their hands) will have to be explained because Allah has no physical hands.
10) Usoolul Fiqh, i.e, Principles of Islamic Jurisprudence. These are necessary for reasoning out and finding arguments in the basic support of statements.
11) Asbaabun Nuzool, i.e, the particular circumstances which caused revelation. The meaning of an ayat will be better understood if we know how and when it had been revealed. Sometimes the true meaning of an ayat is understood only if we know the circumstances in which the ayat had been revealed.
12) An Naasikh wal Mansookh, i.e, knowledge of commandments that have subsequently been abrogated or changed, so that abrogated commandments may be distinguished from the standing ones.
13) Ilmul Fiqh, i.e, knowledge of Islamic Jurisprudence, because it is only through this knowledge that we arrive at a complete understanding of general principles.
14) Knowledge of such Ahadith that happen to be commentary on certain brief verses of the Qur'an.
15) The last but most important is the Wahbi ilm, or the gifted understanding, bestowed by Almighty Allah upon His selected ones, as is referred in the hadith: 'Whosoever acts upon what he knows, Almighty Allah bestows upon him knowledge of things not known to him.'
The branches of knowledge described above are like tools, i.e, essential pre-requisites for a commentator. A commentary written by a person who is not thoroughly acquainted with these branches of knowledge will be based on his personal opinion, which is prohibited. The Sahaba (ra) already had Arabic as their mother tongue, and they reached the depth of the rest of the knowledge by means of their illuminating contact that they had with the Holy Prophet(saw).
'Allamah Suyuti says that those who think that it is beyond the capacity of a man to acquire Wahbi ilm, or gifted understanding, are not right. To get this knowledge from Allah, one should adopt the means to this end, e.g., acting upon the knowledge that one has acquired, and disinclination towards the dunya.
It is stated in 'Keemiyaa-e-Sa'aadat' that three persons are not blessed with complete understanding of the Qur'an. First, one who is not well versed in Arabic, secondly, one who persists in committing a major sin or indulges in acts of religious innovation, because these actions blacken his heart, which in turn prevents him from understanding the Qur'an. Thirdly, one who is a rationalist, even in the matter of faith, and feels embarassed when he reads an ayat of the Qur'an which he is not able to fully rationalize.
Just because someone, no matter how famous, is commentating on the Qur'an already doesn't mean you can assume they know all this. You'd be surprised.
Abdullah ibn Umar (ra) narrated:
"Rasulullah(saw) said, 'The hearts get rusted as does iron with water.' When someone asked, 'What could cleanse hearts again?' Rasulullah(saw) said, 'Frequent remembrance of death and recitation of Qur'an.'"