(Zen wa isoge; “Hurry to do good”)
“Strike while the iron is hot.” Specifically, if you think of something that would be good to do, something worth doing, something worthy – do it now. Don’t wait. Don’t procrastinate. If you have a good idea, a new year’s resolution, a plan – do it now! You can come back and finish reading this post later, I promise. (If your idea is “study more Japanese,” of course, please stay: that’s what you’re already doing.)
善 (zen) is a noun meaning “virtue,” “good(ness).” It’s followed by the topic-marker or emphatic particle は (wa), and the transitive verb 急ぐ (isogu), “to hurry (something)” in imperative form. An obsessively literal rendition might go “As for good(ness), do it quickly.”
The phrase can be extended by adding its inverse, 悪は延べよ (aku wa nobe yo), “delay doing evil.” In this phrasing, the first は should probably be interpreted as emphatic, and the second (the は in 悪は) as contrastive. Finally note that, in Japanese if not in English translation, this saying is considered antonymous with 急がば回れ.
(“Ashita kara, mainichi joggingu suru koto ni shita.” “A, ii ne. Kore kara de mo issho ni shinai? Zen wa isoge da yo.”)
[“I’ve decided that I’ll go jogging every day, starting tomorrow.” “Oh, that’s a good idea! Why not do it now, together? ‘Strike while the iron is hot,’ you know.”]