～と同時に
Two events happen at the same time
You’ll be able to describe simultaneous events or roles and distinguish simultaneous timing from a result that follows immediately afterward.
合図と同時に、子供たちは川へ走りました。
aizu to douji ni, kodomotachi wa kawa e hashirimashita.
What do you think this sentence means?
At the same time as the signal, the children ran toward the river.
At the exact same time
～と同時に means “at the same time as.” It describes two events that occur together, often in formal or written descriptions. Unlike ～たとたん, it focuses on simultaneity.
映画が終わると同時に、みんなが拍手しました。
eiga ga owaru to douji ni, minna ga hakushu shimashita.
Everyone clapped just as the movie ended.
卒業と同時に、東京へ引っ越しました。
sotsugyou to douji ni, toukyou e hikkoshimashita.
I moved to Tokyo right when I graduated.
Connect two events or two roles
Use a dictionary-form verb to present two events as simultaneous, or a past plain verb when narrating two events that occurred together. An event noun can come straight before it: 合図と同時に. To give one person or thing two roles or qualities at once, use Aであると同時にBである.
家を出ると同時に、雨がやみました。
ie o deru to douji ni, ame ga yamimashita.
The rain stopped at the same time as I left the house.
彼は研究者であると同時に、小説家でもあります。
kare wa kenkyuusha de aru to douji ni, shousetsuka demo arimasu.
He is both a researcher and a novelist.
～と同時に vs ～たとたん
Both link two events with no gap. ～と同時に reports them as happening together, in a neutral, often written tone; ～たとたん stresses a sudden, unexpected reaction right after the first event.
～と同時に, at the same time as
店のドアが開くと同時に、犬が入ってきました。
mise no doa ga aku to douji ni, inu ga haitte kimashita.
A dog came in as the shop door opened.
A calm report of two things happening at once.
～たとたん, the instant
店のドアが開いたとたん、犬が入ってきました。
mise no doa ga aita totan, inu ga haitte kimashita.
The instant the shop door opened, a dog came in.
Stresses the surprise: the dog came in the very moment the door opened.
就職と同時に、一人暮らしを始めました。
shuushoku to douji ni, hitorigurashi o hajimemashita.
I started living alone at the same time as I started my job.
朝十時に店が開くと同時に、客がセール品の売り場に集まった。
asa juuji ni mise ga hiraku to douji ni, kyaku ga seeruhin no uriba ni atsumatta.
The moment the store opened at ten in the morning, customers crowded into the sale section.
スマホはとても便利であると同時に、問題も多い道具です。
sumaho wa totemo benri de aru to douji ni, mondai mo ooi dougu desu.
Smartphones are very convenient, but at the same time they are tools with many problems.
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