HSK 7-9
发觉
fājué动
to become aware; to detect; to realize; to perceive
他进门后才发觉拿错钥匙。
Tā jìn mén hòu cái fā jué ná cuò yào shi.
Only after entering did he realize he had taken the wrong key.
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Appears across levels (7-9) in the HSK 3.0 word list.