〜ことだ

Meaning

Expresses advice or counsel that doing so is best. As in 「早く寝ることだ」, a way of recommending the action one should take, out of consideration for the listener.

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, the thing to do is rest well.
合格したければ、毎日こつこつ勉強することだ If you want to pass, the thing to do is study diligently every day.