وَأَصْلُ الْمُبْتَدَأِ أَنْ يَكُونَ مَعْرِفَةً، وَأَصْلُ الْخَبَرِ أَنْ يَكُونَ نَكِرَةً.
Translation: The normal pattern is for the mubtadaʾ to be definite and the khabar indefinite.
Maʿrifa points to something identifiable («زَيْدٌ», «الْكِتَابُ»); nakira denotes an unspecified entity or quality («رَجُلٌ», «مُفِيدٌ»).
«الْكِتَابُ مُفِيدٌ»: definite topic + indefinite predicate. This is the default, not an absolute ban on other constructions.
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Mubtadaʾ and Khabar — Definition and Foundational Rules
وَأَصْلُ الْمُبْتَدَأِ أَنْ يَكُونَ مَعْرِفَةً، وَأَصْلُ الْخَبَرِ أَنْ يَكُونَ نَكِرَةً.
Translation: The normal pattern is for the mubtadaʾ to be definite and the khabar indefinite.
Maʿrifa points to something identifiable («زَيْدٌ», «الْكِتَابُ»); nakira denotes an unspecified entity or quality («رَجُلٌ», «مُفِيدٌ»).
«الْكِتَابُ مُفِيدٌ»: definite topic + indefinite predicate. This is the default, not an absolute ban on other constructions.