We cannot translate it by a word. Depending on the sentence, we need to change it. Let’s see with example sentences.
一日ずつ
いちにちずつ
ichinichi zutsu
- どうぶつの森は、日にちを変えないで一日ずつやりましょう！
doubutsunomori wa, hinichi wo kaenaide ichinichi zutsu yarimashou!
You should play Animal Crossing day by day without changing the dates.
- 一日ずつ。(しっかりと生きて行こう！)
ichinichi zutsu. (shikkarito ikiteikou!)
*inside the parenthesis is an implication.
One day at a time.
Un jour à la fois.
This phrase is not the only translation of ‘One day at a time’, however when I felt stressed about immigrating to Canada (to a French speaking area. I could not speak any French at all at that time.) and I cried, my mother-in-law gave me the phrase ‘Un jour à la fois’. In that situation, I understood that she said the sentence as I wrote above.
一日一歩
いちにちいっぽ
ichinichi ippo
one day one step
This is one of the 四字熟語 (よじじゅくご / yoji jukugo = Four character idiomatic compounds).
- 一日一歩、成長していく。
ichinichi ippo, seichou shiteiku.
Growing (improving) day by day.
日に日に
ひにひに
hinihini
- 歯医者さんの英語が、日に日に良くなっていますね！
haisha-san no eigo ga, hinihini yokunatte imasune!
Haisha-san(my beaver plush doll), your English is getting better day by day.
- 親知らずのせいで、歯並びが日に日に悪くなる。
oyashirazu no seide, hanarabi ga hinihini warukunaru.
Because of the wisdom teeth, my teeth alignments get worse day by day.
