رَأَيْتُ هَذَا الطَّالِبَ.
Translation: I saw this student.
رَأَيْتُ: past verb + subject tāʾ. هَذَا: demonstrative, indeclinable, in the accusative position as object. الطَّالِبَ: identifying apposition, accusative with fatḥa.
The demonstrative remains unchanged, even though its syntactic position is accusative.