不由得: can't help (doing), involuntarily
【高25】副词 / adverbs
Tone colorsā tone 1á tone 2ǎ tone 3à tone 4a neutral
不由得 + verb says that a reaction escapes the subject's will, set off by what he sees, hears or remembers: "can't help (doing), in spite of oneself". The trigger comes first (看着父亲粗糙的手, 听到那首老歌), then the reaction: 流下了眼泪, 笑了起来, 想起往事, 加快了脚步.
It is used mostly in narration, in the past, with 了 or a complement such as 起来. What follows is a verb of reaction (laugh, cry, remember, think, sigh) or a change of state (难过起来, 紧张起来), never a decided action. In speech 忍不住 is more usual; 不由得 is more written and more narrative.
This page teaches the adverb. 不由得 also exists as a verb, "not to let, not to allow" (不由得你不信: you cannot help believing it), which is a different use.
Examples
听到这个笑话，大家不由得笑了起来。
Tīng dào zhège xiàohua, dàjiā bùyóude xiào le qǐlái.Hearing the joke, everyone could not help laughing.
看着父亲粗糙的手，我不由得流下了眼泪。
Kàn zhe fùqin cūcāo de shǒu, wǒ bùyóude liúxià le yǎnlèi.Looking at my father's rough hands, I could not help shedding tears.
走进熟悉的老街，他不由得放慢了脚步。
Zǒu jìn shúxi de lǎojiē, tā bùyóude fàngmàn le jiǎobù.Walking into the familiar old street, he found himself slowing down.
看着毕业照，我不由得回忆起往事。
Kàn zhe bìyèzhào, wǒ bùyóude huíyì qǐ wǎngshì.Looking at the graduation photo, I could not help recalling the past.
一想到明天就要见到母亲，我不由得兴奋起来。
Yì xiǎng dào míngtiān jiù yào jiàndào mǔqin, wǒ bùyóude xīngfèn qǐlái.At the thought of seeing my mother the next day, I grew excited in spite of myself.
站在站台上，望着远去的列车，他不由得叹了一口气。
Zhàn zài zhàntái shang, wàng zhe yuǎnqù de lièchē, tā bùyóude tàn le yì kǒu qì.Standing on the platform and watching the train pull away, he could not help sighing.
Common mistakes
✗ 我不由得去商店买东西 → ✓ 我不由得笑了 (what follows must be an involuntary reaction, not a decided action).
✗ 我不由得很难过 → ✓ 我不由得难过起来 (use a reaction verb or a change of state with 起来 or 了, not a plain state with 很).