A line from a song lyric: "人理所当然的忘记"
人 - people (subject)


理所当然的 - naturally (adverbial phrase)


忘记 - forget (verb)


理 (logic)

理所 (according to logic)

当然 (of course; certainly)

理所当然 = naturally; of course

理所当然的 = naturally; of course