"I forgot to return the book to the library."

Translation: 図書館に本を返すのを忘れました。

[Player1UK]

I've come across のは and のが and other forms, but not のを before.

Can somebody explain the difference, please?

[gtphil]

The の nominalizes the verb making it act as a noun. The subsequent particle indicates the role of the nominalized verb. を makes 'to return the book' the object of the verb 'forget'.

[darthoctopus]

図書館に本を返すのを忘れました

[Jules40223]

Would this sentence more directly translate as: I forgot about returning the book to the library?

[laters-taters]

I like that 忘 = 亡+心 (forget = die+mind)

[joseduc]

why is this wrong? 図書館に本を返すのを忘れました

[Enji_Todoroki]

It's been fixed and accepted as of 9.4.20

[5Ozuduu0]

I am confuse here. What if there is just one "wa" used here? How will that change?

[7jeny3]

Do you mean "wo"? There is no "wa" here.

[Dylan_Nicholson]

Can't が be used in place of を here?

[laters-taters]

忘れる is a transitive verb just like “forget”, so it takes a direct object (the thing being forgotten, in this case 図書館に本を返すの), so it's always marked with を.