nominalizer N3 common casualpolitewritten

連用形の名詞用法 — using a verb's masu-stem as a noun: 動く→動き 'movement', 遊ぶ→遊び

連用形の名詞用法・れんようけいのめいしようほう

Meaning

the ます-stem used as a standalone noun: 動うごき 'movement', 遊あそび 'play', 始はじまり 'beginning'

波なみの動うごきを観察かんさつする。

I observe the movement of the waves.

Attaches to	Form	Example
Verb (ます-stem)	V-stem (used as a noun)	動うごく → 動うごき; 遊あそぶ → 遊あそび; 帰かえる → 帰かえり

子こどもたちは遊あそびに夢中むちゅうだ。

The children are absorbed in play.

物語ものがたりの始はじまりはとても静しずかだった。

The beginning of the story was very quiet.

彼かれの話はなしには深ふかい悲かなしみがあった。

There was a deep sadness in what he said.

Not every verb yields a common stem-noun, and meanings can drift (話はなす → 話はなし 'a story/talk', not just 'speaking'). Productive ones are learned as vocabulary; treat this as a pattern to recognize, not to coin freely.

Jengo shows 連用形の名詞用法 the way you actually meet it: inside real Japanese sentences, so it sticks instead of staying an abstract rule.

Sources Compiled with reference to A Dictionary of Intermediate Japanese Grammar.