( )Upon __ his homework,Joe ran to the living room to watch cartoons.(1)finished(2)finish(3)finishes(4)finishing?

upon是「介系詞」，後面空格裡應該加「動名詞」finishing當作其「受詞」。
upon finishing his homework是「介系詞片語」，用來修飾後面的述部，提供「時間點」。

finish=verb
eg:-Joe put the finishing touches to his painting
=Joe did the things that made it complete the painting.


Having finished his homework=Upon finishing his homework.
Upon is formal preposition followed by present participle.
Upon finishing his homework=prep. phrase qualifying main clause as manner ,condition.

Upon (4)finishing his homework, Joe ran to the living room to watch cartoons.
(4)finishing=(grammar) a word formed from a verb, ending in --ing(=the present participle as adjective)
Or ending in ---ed, ---en etc(=the past participle as adjective)

Upon為介係詞『一~就~』的意思,後面要接V-ing(動名詞),因此答案為(4),而finishing的動作者就是Joe

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