判断・推量N3
〜ことだ
should... (advice) / it's important to...
Structure
V-辞書形 / V-ない形 + ことだ
Explanation
Attached to the dictionary/nai-form of verbs, ことだ gives advice or a recommendation, "should...". More direct and colloquial than べきだ (疲れた時は、よく休むことだ).
Examples
疲れた時は、ゆっくり休むことだ。
When you're tired, you should rest.
上達したければ、毎日練習することだ。
If you want to improve, you should practice every day.
Note
ことだ gives advice, often from a senior; compared with べきだ it emphasizes "this is what you should do".