と同時(どうじ)にAt the same time as, The instant, The moment, As well as

とき expresses the general time of an action or event, like 'when' in English. と同時に means 'at the same time as' and emphasizes simultaneity. For example, 帰ってきた時 means 'when (I) came home', while 帰ってきたと同時に means 'the moment (I) came home' or 'as soon as (I) came home'.

Both と同時に and なり follow the plain form of verbs and describe events occurring in close succession, but their nuance differs. と同時に means 'at the same time as' and emphasizes actual simultaneity. In contrast, なり highlights immediate consequence, something happening as soon as the first action occurs, and often implies a reflexive or automatic response.

Both と同時に and たとたんに describe closely linked events, but they differ in focus. と同時に emphasizes simultaneity or coordination, while たとたんに conveys a sudden, often unexpected result following a completed action. Their grammar structure also differs: と同時に follows the non-past form, while たとたんに follows the past tense.

でもある and と同時に can both express the idea of 'also' or '(A) as well as (B)', though they function differently. と同時に follows the first item of information, while でもある follows the additional information. Because they appear in different parts of a sentence, they can also be used together. For example, ピアニストであると同時に、作曲家でもあります ('In addition to being a pianist, he is also a composer').

と同時に and と共に can both mean 'together with' or 'at the same time as', but their nuance differs. と同時に emphasizes temporal simultaneity in neutral or factual contexts, while と共に often conveys shared action, emotional resonance, or development over time.