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の、忘れないでよ

don't forget to

→ の
phrase Rare #40,761
don't forget to A casual reminder or request. の nominalizes the action before it, and よ adds emphasis; the phrase can sound mildly insistent depending on tone.
JA: 明日の朝、鍵をかけるの、忘れないでよ。
EN: Don't forget to lock the door tomorrow morning.

タバコの火を消すの、忘れないでよ。

Don't forget to put out your cigarette.

窓を閉めるの、忘れないでよ。

Don't forget to close the window.

図書館に本を返すの、忘れないでよ。

Don't forget to return the book to the library.

ドアを閉めるの、忘れないでよ。

Don't forget to close the door.