expression
Fixed expression used to remark on how quickly time has passed, often at the beginning of a sentence when reflecting on a past event or milestone. The literal meaning is 'it's a fast thing', but it functions as a set phrase.
早いもので、日本に来てからもう一年が経ちました。
Time flies—it's already been a year since I came to Japan.
早いもので、今年もあと一ヶ月ですね。
Time flies—there's only one month left in this year, isn't there?
もう simply means 'already' and is a common adverb. 早いもので is a set phrase that emphasizes the subjective feeling that time has passed quickly.
A classical proverb meaning 'time flies like an arrow'. 早いもので is a more colloquial, everyday expression for the same sentiment.
A fixed phrase composed of 早い (fast/early), もの (thing), and the conjunctive particle で. The literal sense 'it being a fast thing' developed into an idiomatic expression for the swift passage of time.