借りたお金は返すべきだと思います。
- I think borrowed money should be returned.

借りたお金は返すべきだと思います。 meaning: I think that you should return the money you borrowed.

⚫︎ 借りた is the past-tense modifier form of the verb 借りる (to borrow). ⚫︎ It modifies the noun お金 (money) to mean "the money that was borrowed." ⚫︎ This structure is a standard Japanese relative clause where a verb directly precedes a noun without a relative pronoun.

⚫︎ The particle は marks 借りたお金 as the topic of the clause. ⚫︎ It highlights the borrowed money as the main subject being discussed. ⚫︎ It sets the context for what should be done with that money.

⚫︎ べきだ is an N2 grammar point used to express moral obligation, duty, or strong advice. ⚫︎ It attaches directly to the dictionary form of a verb, as seen in 返すべき. ⚫︎ It conveys the speaker's judgment that something is the right and natural thing to do.

⚫︎ 返す (to return) is a Type 1 (Godan) verb. ⚫︎ To attach べき, the verb changes to its terminal/dictionary form 返す. ⚫︎ Note that the exception verb する can become either するべき or すべき.

⚫︎ と思います is an essential expression used to state the speaker's personal opinion or thought. ⚫︎ It consists of the quotation marker と and the verb 思う (to think) in the polite ます form. ⚫︎ It softens the assertion 返すべきだ into a subjective opinion rather than an absolute fact.