Three Readings, One Idea
と共に means "along with", and which sense applies depends on what precedes it.
1. Together with a person.
- 家族と共に暮らしています。(I live together with my family.)
2. At the same time as an event.
- 卒業と共に上京した。(I moved to Tokyo upon graduating.)
3. As one thing changes, so does another.
- 年を取ると共に考え方が変わった。(My thinking changed as I got older.)
Sense 3 vs につれて
Both cover parallel change, and they are close enough to swap in most sentences:
| Pattern | Nuance |
|---|---|
| につれて | gradual, continuous |
| と共に | can be gradual or simultaneous at a single moment |
卒業と共に works because it is one moment. ×卒業につれて does not, because there is no gradual process.
Against と一緒に
と一緒に is the everyday word for sense 1. と共に is its formal counterpart and additionally carries senses 2 and 3, which と一緒に cannot.
Register
Formal and written. In speech, use と一緒に for people and につれて for change.