我突然发觉自己忘记了带钥匙。

wǒ tūrán fājué zìjǐ wàngjì le dài yàoshi.

I suddenly realized that I forgot to bring the keys.

他在比赛中发觉对手的弱点。

tā zài bǐsài zhōng fājué duìshǒu de ruòdiǎn.

He noticed the weakness of his opponent during the competition.

她发觉自己对这个问题一无所知。

tā fājué zìjǐ duì zhège wèntí yīwúsuǒzhī.

She realized that she knew nothing about this issue.