こと details
write + do — a rule or instruction set down in writing
kaku koto
do — a rule or instruction set down in writing
meaning: write
使った道具は、必ず元の場所に戻すこと。
tsukatta dougu wa, kanarazu moto no basho ni modosu koto.
Always return tools you have used to the place they came from. formal
会場では、携帯電話の電源を切ること。
kaijou de wa, keitai denwa no dengen o kiru koto.
Mobile phones are to be switched off inside the hall. formal
Examples
do — a written instruction, in rules, notices, syllabuses and homework directions
使った道具は、必ず元の場所に戻すこと。
tsukatta dougu wa, kanarazu moto no basho ni modosukoto.
Always return tools you have used to the place they came from. formal
会場では、携帯電話の電源を切ること。
kaijou de wa, keitai denwa no dengen o kiru koto.
Mobile phones are to be switched off inside the hall. formal
レポートは金曜日の正午までに提出すること。
repooto wa kin'youbi no shougo made ni teishutsu suru koto.
Reports are to be handed in by noon on Friday. formal
do not do — the same written instruction aimed at what is forbidden
実験室に飲み物を持ち込まないこと。
jikkenshitsu ni nomimono o mochikomanai koto.
Drinks are not to be brought into the laboratory. formal
冷蔵庫のケーキ、勝手に食べないこと！
reizouko no keeki, katte ni tabenai koto!
The cake in the fridge is not to be eaten without asking! casual
Formation
{V}ことattaches to dictionary form{V-nai}ことattaches to ない-form
Notes
back into advice. Read out to someone present it lands as reciting the rules
at them.
Related patterns
Similar
なさい keeps a speaker with standing over the reader — a parent, a teacher, an exam paper; こと has no speaker at all, which is why house rules and notices take it and reading it to someone's face sounds like reciting the regulations at them.
べきだ argues that the act is the right one and invites disagreement (editorials run on it); こと states it as already settled and gives no ground to argue on, so it belongs to rule sheets rather than to opinion.
こと posts the rule at nobody and takes no particle after it; ように is still aimed at a person, which is why it accepts ね and な — こと never can, printed or spoken, and both turn up in a posted notice.
Lookalikes
Same こと on the same dictionary form, but the noun-making one needs a predicate after it (走ることが好きだ); the instruction closes the sentence there, so a こと with nothing following is an order rather than a noun.
Drop the だ and the speech act changes — ことだ hands blunt advice to someone in front of you, bare こと is posted where nobody is present; a learner who adds です to a notice turns a regulation back into advice.