godan verb (-tsu), transitive verb
make up one's mind; resolve to do; hit upon the idea
Expresses the moment of deciding or getting the idea to do something, often suddenly or on impulse. Commonly used in the set phrase 思い立ったが吉日 (strike while the iron is hot).
彼は急に旅行に行こうと思い立った。
He suddenly made up his mind to go on a trip.
思い立
There's no time like the present; I'll start studying today.