How to tell them apart
と共に emphasizes that A and B share a property or act together, often in a lasting or inherent sense, while と同時に stresses that two actions or states occur at the same moment. Swapping them can sound unnatural: 建設的であると共に革命的だ works because the two qualities coexist, but 建設的であると同時に革命的だ highlights simultaneity rather than shared nature.
- と共に Use と共に when two things share a property or act together in an ongoing, inherent way.
- と同時に Use と同時に when two actions or states happen at the same moment or simultaneously.
| 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 follows a noun and means that something happens together with, or at the same time as, that noun. It can link two things that develop side by side, describe accompanying someone, or mark the moment something occurs, as in rising with the sunrise. The tone is somewhat formal or written.
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身体は精神と共に発達しなければならない。
The body must be developed along with the mind.
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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 means that two things happen at the same time, or that something happens the instant another thing happens. It attaches to a plain-form verb, a noun, or a full clause, and can also follow それ or これ as それと同時に to link two events.
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私たちがプラットホームに着くのと同時に電車が出てしまった。
The train left just as we arrived at the platform.
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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.
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.