～ことだ

How to form it

• Verb (dictionary form) + ことだ
• Verb (ない-form) + ことだ

Register: Neutral; advice from someone of equal or higher standing

Explanation

Gives advice about what the listener should do. The advice is specific to the situation.

Sounds a little like a teacher or an older person giving guidance, so it is not for talking to a superior.

Examples

1. 日本語が上手になりたいなら、毎日話すことだ。

   にほんごがじょうずになりたいなら、まいにちはなすことだ。

   If you want to get good at Japanese, you should speak every day.

2. 風邪をひいたら、ゆっくり休むことだ。

   かぜをひいたら、ゆっくりやすむことだ。

   If you catch a cold, the thing to do is rest properly.

3. 合格したければ、もっと勉強することだ。

   ごうかくしたければ、もっとべんきょうすることだ。

   If you want to pass, you need to study more.

Compare with

• ～ものだ: ものだ states a general norm; ことだ gives direct advice.
• ～に限る: に限る says one thing is best; ことだ tells someone what to do.

Common mistakes

• Using it to advise a teacher or a boss. It can sound presumptuous.