مَرَرْتُ بِالْعَصَا: I passed by the stick.
مَرَرْتُ: past verb, built on sukūn, with subject tāʾ.
بِـ: preposition.
الْعَصَا: genitive noun governed by bāʾ, with estimated kasra on final alif due to impossibility.
Unlike the earlier sentences, this noun follows a preposition.