〜ことだ
(Advice) should, the thing to do ismachine translation
FormationVerb dictionary form / ない-form + ことだ
〜ことだ is direct advice aimed at ONE person in ONE specific situation: 風邪を早く治したいなら、ゆっくり休むことだ (if you want to get over that cold quickly, you should get plenty of rest). The speaker presents what they consider the best course of action, so this pattern is typically used by superiors speaking to subordinates, or between close friends.
Connection: verb dictionary form + ことだ (advising to do), verb ない-form + ことだ (advising not to do): 心配しないことだ (don't go worrying about it). The polite form is ことです.
Compared with 〜べきだ: べきだ speaks of general duty and morality ("as a person, one must…"), while ことだ targets only the listener's own benefit in the situation at hand. And unlike 〜ほうがいい, which you learned earlier — a gentle suggestion — ことだ sounds as decisive as "there's only one way," so don't use it with superiors.
Examples
風邪を早く治したいなら、ゆっくり休むことだ。
If you want to get over your cold quickly, you should get plenty of rest.
合格したければ、毎日勉強することです。
If you want to pass, you need to study every day.
心配しないことだよ。きっとうまくいくから。
Don't worry about it. I'm sure it'll all work out.