の is used when the speaker describes the situation as his concrete thing. こと is used when the speaker describes the situation as an abstract thing. 泳ぐのが好きです。correct　※The speaker says this sentence while remembering himself /herself swimming. 泳ぐことが好きです。correct　※The speaker says this sentence while thinking that my hobby is swimming. 甘いものを食べるのは体に悪いです。correct　※The speaker says this sentence while remembering his /her/someone's concrete example caused by eating sweets. 甘いものを食べることは体に悪いです。correct　※The speaker says this sentence while thinking that it is said that eating sweets is bad for our health. 趣味は将棋をすることです。correct　※The speaker says this sentence while thinking that my hobby is to play shogi. 趣味は将棋をするのです。incorrect　※This form is not used because there are another grammatical idiom "のです". 辞書を持ってくることを忘れました。incorrect when this sentence is used to describe the speaker's concrete action. When you use "ことを忘れる" in an abstract/ordinary situation, it's correct. 彼は私が彼にお金を貸したことを忘れたと言っている（He has said that he has forgotten that I lent him money）。correct　※"お金を貸したこと" is used as the general meaning "lending money". 辞書を持ってくるのを忘れました。correct 彼は私が彼にお金を貸したのを忘れたと言っている（He has said that he has forgotten that I lent him money）。correct　※"お金を貸したの" is used to express the concrete/personal situation when the speaker lent him money". You can also see the next sites. https://www.jpf.go.jp/j/project/japanese/teach/tsushin/grammar/201112.html https://www.jpf.go.jp/j/project/japanese/teach/tsushin/grammar/201203.html http://www.lib.tezuka-gu.ac.jp/kiyo/nTEZUKAYAMAGAKUIN-UNI/n8PDF/n8Kawagoe.pdf http://eprints.lib.hokudai.ac.jp/dspace/bitstream/2115/45666/1/BISC010_008.pdf

It will explain that results after "V +の" . So..."V +の"+が or は or を (particle)etc. It is next to results sentence. But "V +こと" is next to sentence "です"or"でした" It will be finished. In more detail, "V +こと" = a noun.

It will explain that results after "V +の" . So..."V +の"+が or は or を (particle)etc. It is next to results sentence. But "V +こと" is next to sentence "です"or"でした" It will be finished. In more detail, "V +こと" = a noun.