رَأَيْتُ الْعَصَا: I saw the stick.
رَأَيْتُ: past-tense verb, built on sukūn because it is attached to subject tāʾ.
تُ: attached subject pronoun in the nominative position.
الْعَصَا: accusative direct object with estimated fatḥa on final alif, hidden due to impossibility.
The same noun was a subject in sentence 1; here it is an object, with no visible change.