expression
at first; initially; in the beginning
Refers to the early stage of a period or situation, often implying that things changed later. Commonly used with past-tense narratives.
初めのうちは緊張していたが、だんだん慣れてきた。
I was nervous at first, but I gradually got used to it.
初めのうちは雨が降っていたが、午後には晴れた。
It was raining in the beginning, but it cleared up in the afternoon.
最初 is a noun/adverb meaning 'first' or 'beginning', often used for the very first moment or item. 初めのうち focuses on the early phase of a period, not a single point.
初めは is a simpler adverbial phrase meaning 'at first'. 初めのうち adds a nuance of 'during the early part' and often implies a longer initial period.
A set phrase combining 初め (beginning) + の (possessive particle) + うち (inside/period). The うち here indicates a time frame, giving the meaning 'during the early part'.