彼の忠告を

his advice

→ 彼
phrase Rare #37,105
his advice The particle を marks this phrase as the direct object of a verb.
JA: 彼の忠告を忘れないでください。
EN: Please don’t forget his advice.

彼の忠告を聞かなかったことを後悔してるよ。

I regret not listening to his advice.

私たちは彼の忠告を聞くべきだった。

We should have listened to his advice.