The following illustrations require attention not only to al- but also to context, meaning, and the domain of predication.
ExpressionIntended interpretation
رَأَيْتُ رَجُلًا فَسَأَلْتُ الرَّجُلَExternal reference: the man already mentioned
اُدْخُلِ السُّوقَMental reference: an appropriate market
الإِنْسَانُ حَيَوَانٌ نَاطِقٌGeneric essence: the nature of human beings
إِنَّ الْإِنْسَانَ لَفِي خُسْرٍExhaustive inclusion: broadly inclusive, then qualified by exception
The first and third are pedagogical illustrations, while the second and fourth reflect the source's central interpretive problems. The governing principle is that one mark of definiteness can serve several distinct rhetorical ends.