I've come across のは and のが and other forms, but not のを before.
Can somebody explain the difference, please?
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'.
Would this sentence more directly translate as: I forgot about returning the book to the library?
I like that 忘 = 亡+心 (forget = die+mind)
why is this wrong? 図書館に本を返すのを忘れました
It's been fixed and accepted as of 9.4.20
I am confuse here. What if there is just one "wa" used here? How will that change?
Do you mean "wo"? There is no "wa" here.
Can't が be used in place of を here?
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.