مَرَرْتُ بِهَذَا الطَّالِبِ.
Translation: I passed by this student.
مَرَرْتُ: verb + subject. بِـ: preposition. هَذَا: indeclinable demonstrative in a genitive position. الطَّالِبِ: apposition, genitive with kasra.
Distinguish the demonstrative’s syntactic position from the following noun’s visible case ending.