Translation guide
The English word "act" covers a wide range of meanings, from taking action to performing in a play, behaving in a certain way, or serving a function. This guide breaks down the main uses and provides natural Japanese equivalents for each.
To do something, especially in response to a situation or to achieve a result.
The most general and common way to say "act" in the sense of taking action. It implies moving or behaving with a purpose.
すぐに行動する必要がある。
We need to act immediately.
彼は言葉ではなく行動で示した。
He acted rather than just talking.
Literally "to move," often used when someone finally takes action or when a group/organization starts doing something.
To take measures or countermeasures; used when acting to solve a problem.
政府は早急に対策を取るべきだ。
The government should act quickly.
To behave or conduct oneself, often with an adverb describing the manner.
To perform a role in a dramatic production.
To behave as if something is true when it is not; to fake an emotion or role.
The most common way to say "pretend" or "act like." Used with verbs or adjectives.
彼は病気のふりをした。
He acted sick.
知らないふりをしないで。
Don't act like you don't know.
To fulfill a purpose or role, often for objects or systems.
To function as; used for machines, systems, or abstract things.
この装置はセンサーとして機能する。
This device acts as a sensor.
To start working or become operative.
To be effective; used for medicine, poison, magic, etc.
A thing done; an act as a countable noun.
A general term for an act or deed, often used in formal or legal contexts.
A major section of a theatrical work.
Act of a play; also used metaphorically for stages of life.
行動する focuses on taking concrete actions, while 振る舞う describes the manner of behavior or conduct. Use 行動する for "act decisively" and 振る舞う for "act politely/rudely."
彼は勇敢に行動した。
He acted bravely (took brave actions).
彼は勇敢に振る舞った。
He acted bravely (behaved in a brave manner).
English often uses "act" as a noun where Japanese prefers a verb phrase. For example, "an act of kindness" is more naturally 親切な行為, but in many cases rephrasing with a verb is better.
子供みたいなふりをするのはやめて。
Stop acting like a child.
その薬は脳に作用する。
The drug acts on the brain.
The police haven't acted yet.
Describes how someone behaves, especially in social situations. Can be positive or negative.
彼は紳士のように振る舞った。
He acted like a gentleman.
子供たちは行儀よく振る舞っていた。
The children were acting politely.
Also used for behaving, but more focused on actions than mannerisms.
彼は奇妙な行動をした。
He acted strangely.
Pattern: "act like a ~"
彼女は女王のように振る舞う。
She acts like a queen.
To play a role; the standard verb for acting in theater, film, or TV.
彼はハムレットを演じた。
He acted Hamlet.
彼女はその映画で主役を演じている。
She acts the lead role in that movie.
To perform acting; emphasizes the craft of acting itself.
彼はとても自然に演技する。
He acts very naturally.
To appear in a production; focuses on being in the cast rather than the performance.
彼女は来月の舞台に出演する。
She will act in next month's play.
To feign or pretend, often used for emotions or states. Slightly more formal/literary.
彼女は平静を装った。
She acted calm.
Can also mean to put on an act in real life, implying insincerity.
彼の悲しみは演技だった。
His sadness was just an act.
To work or act in a certain way, often for forces or influences.
重力がブレーキとして働く。
Gravity acts as a brake.
To play a role; emphasizes fulfilling a function or duty.
教育は重要な役割を果たす。
Education acts an important role.
この薬はすぐに効く。
This medicine acts quickly.
To act on something; more technical, used in science or formal contexts.
この化学物質は神経に作用する。
This chemical acts on the nerves.
To come into effect (for laws, treaties).
新しい法律が来月発効する。
The new law will act next month.
それは犯罪行為だ。
That is a criminal act.
彼の親切な行為に感謝します。
I appreciate his kind act.
Action or behavior; more about the process than a single deed.
彼の行動は理解できない。
I can't understand his acts.
A deed, often with a nuance of mischief or blame. "The work of..."
これは誰の仕業だ？
Whose act is this?
第二幕が始まる。
The second act begins.