〔欠くことのできない〕essential; the most important ((person; thing))
- 練習を続けることが肝心だ
- It is essential that you keep on practicing.
- 肝心な点は彼がその時そこにいたかどうかだ
- The main [crucial] point is whether he was there at that time or not.
- 肝心な時に病気になってしまった
- I was taken ill at the critical moment.