次第に（しだいに）～

About

We can combine 次第しだい and the に particle to form an adverb that modifies a following verb and indicates that it happens "gradually".

The typical English approximations of 次第しだいに～ are "gradually" or" slowly".

次第しだいに～ is most commonly used in polite speech/written scenarios.

E.g. 毎日まいにちプランクを続つづけるうちに、次第しだいに腹筋ふっきんが割われてきた。 "As I continued to do planks every day, my abs gradually developed. "

E.g. 朝あさは晴天せいてんだったが、次第しだいに風かぜと雨あめが強つよまってきた。 "The weather was nice this morning, but the wind and rain gradually grew stronger."

E.g. ATMの利用りようは、次第しだいに減へっているようだ。 "Apparently, ATM use is gradually declining."

Summary

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  次第しだいに～ is an adverb that means "gradually" or" slowly"