ことだ details
write + the thing to do is
kaku koto da
the thing to do is; you'd best do
meaning: the thing to do is write
焦らず、少しずつ慣れることだ。
aserazu, sukoshi zutsu nareru koto da.
Don't rush — the thing to do is get used to it little by little. neutral
そんなに結論を急がないことだよ。
sonnani ketsuron o isoganai koto da yo.
Don't rush to a conclusion like that. casual
- polite
Examples
the thing to do is; you'd best — blunt, prescriptive advice, often senior-to-junior or self-directed
焦らず、少しずつ慣れることだ。
aserazu, sukoshi zutsu narerukotoda.
Don't rush — the thing to do is get used to it little by little. neutral
文句があるなら、直接本人に言うことだよ。
monku ga aru nara, chokusetsu honnin ni iukotoda yo.
If you've got a complaint, the thing to do is tell the person directly. casual
何よりもまず、専門家に相談することです。
nani yori mo mazu, senmonka ni soudansurukotodesu.
More than anything, the thing to do is consult a specialist. neutral
the thing NOT to do is; don't — the same blunt advice aimed at what to avoid
そんなに結論を急がないことだよ。
sonnani ketsuron o isoganai koto da yo.
Don't rush to a conclusion like that. casual
個人情報をむやみに教えないことです。
kojin jouhou o muyami ni oshienai koto desu.
The thing not to do is hand out your personal information carelessly. neutral
Formation
{V}ことだattaches to dictionary form{V-nai}ことだattaches to ない-form
Notes
offering it, which is why it suits a teacher, parent, or senior more than a
peer speaking to an equal. Rarely takes 〜ね; よ is the natural tag instead.
Related patterns
Similar
べきだ argues from principle and freely takes institutional or third-party subjects (政府[せいふ]は事実[じじつ]を説明[せつめい]すべきだ); ことだ needs an addressee and a controllable act pointed at them or the speaker, so the same institutional subject fails (×政府[せいふ]は事実[じじつ]を説明[せつめい]することだ) where すべきだ succeeds.
たほうがいい scales advice to the listener's one situation and stays hedgeable; ことだ states the answer as if it were simply obvious, closer to a flat directive than a suggestion — it takes よ readily but rarely softens with ね the way たほうがいい does.
Lookalikes
Same こと frame, near-opposite force — ことだ prescribes an action ("the thing to do is…"), while ことはない waives one ("no need to…") — skimming past だ／はない swaps a directive for a reassurance.
One こと apart in the wrong place to skim past — ということだ closes a REPORT or conclusion (中止[ちゅうし]ということだ "that means it's canceled"), while ことだ is a bare advisory imperative built straight on the verb (早[はや]く行[い]くことだ "you'd better go soon") — dropping the leading という changes what kind of sentence you're even in.