How to tell them apart
と共に means together with, both A and B, or that one change progresses along with another, as in 友人と共に暮らす or 年を取ると共に視力が衰える. と同時に means two events happen at exactly the same moment, as in ベルが鳴ると同時に授業が終わった. For inherent dual qualities, 建設的であると共に革命的だ is natural, but 建設的であると同時に革命的だ is unnatural because と同時に needs a punctual moment, not lasting coexistence.
- と共に Use と共に to say you act together with someone, that something has two properties at once, or that one change happens along with another.
- と同時に Use と同時に to say two events happen at exactly the same moment.
| Grammar point | Meaning | JLPT level |
|---|---|---|
| と共に to ni | in company with someone or something; at the same time as; in the course of | N3 |
| と同時に to ni | simultaneously with; the moment something happens | N3 |
と共に to ni
in company with someone or something; at the same time as; in the course of
と共に expresses doing something together with a person or thing, or that one change happens at the same time as another, as in 年とともに. It attaches to a noun as N + と共に, and to a verb in dictionary form as V-辞書形 + と共に. It is formal and common in writing, so in casual speech と一緒に is more natural and と共に sounds stiff in direct commands.
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彼は私とともに喜んでくれた。
He shared in my happiness.
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あなたと共に人生を歩んでいきたい。
I want to spend my life with you.
と同時に to ni
simultaneously with; the moment something happens
と同時に expresses that two events happen at exactly the same time, or that a second action starts the instant the first occurs. It follows a noun, often in the pattern Noun + と同時に, and for identity or role it can follow である, as in 科学者であると同時に. It also follows the dictionary form of a verb, as in 着くと同時に. The expression is formal and common in writing and news, so it sounds stiff in casual conversation.
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これと同時に次のことを考えておく必要がある。
It's necessary to think about both this one and the next one at the same time.
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彼が入ってきた、それと同時にベルが鳴った。
He came in, and at the same time the bell rang.
と共に and と同時に are both JLPT N3 grammar points that Japanese learners often mix up. This page compares them side by side on meanings, usage and example sentences, so you can tell them apart.