«هَذَا الطَّالِبُ»: this student; «هَذَا» is a demonstrative.
«الطَّالِبُ الَّذِي نَجَحَ»: the student who succeeded; «الَّذِي» is relative and needs «نَجَحَ» as its ṣilah.
A demonstrative points out a referent; a relative noun completes identification through a clause.
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Relative Nouns — Ṣilah, ʿĀʾid, Forms and Syntax
«هَذَا الطَّالِبُ»: this student; «هَذَا» is a demonstrative.
«الطَّالِبُ الَّذِي نَجَحَ»: the student who succeeded; «الَّذِي» is relative and needs «نَجَحَ» as its ṣilah.
A demonstrative points out a referent; a relative noun completes identification through a clause.