suddenly; all of a sudden
忽 (Hū) · 8 strokes
Phonosemantic compound. 心 represents the meaning and 勿 represents the sound.
然 (rán) · 12 strokes
Phonosemantic compound. 灬 represents the meaning and 肰 represents the sound. Based on the original meaning "burn", now written as 燃. The current meaning is a phonetic loan.