The adverb 不由得 "involuntarily"
The adverb 不由得 
is used to indicate that an action is done against someone’s will.
不由得 is placed after the subject and before a verb used as a predicate.
is used to indicate that an action is done against someone’s will.
不由得 is placed after the subject and before a verb used as a predicate.Examples
看着父亲粗糙的手，我不由得流下了眼泪
Looking at my father’s rough hands, I couldn’t hold back my tears
kànzhe fùqīn cūcāo de shǒu, wǒ bùyóude liúxià le yǎnlèi
他不由得发出一声呻吟
He groaned involuntarily
tā bùyóude fāchū yīshēng shēnyín
看着毕业照，我不由得开始回忆起往事
When I look at graduation photos, I unwittingly remember the past
kànzhe bìyè zhào, wǒ bùyóude kāishǐ huíyì qǐ wǎngshì