Translation guide
Expressing a duration of one year or a single year in time.
for one year (duration)
one year (calendar)
one year old
Expressing a period of one year.
Standard way to say 'for one year' as a duration. The suffix 〜間 emphasizes the span of time.
I lived in Japan for one year.
I'm planning a one-year study abroad.
Can mean 'one year' as a duration, but without 〜間 it may sound more like a point in time or a general count. Often used in compounds or when the duration is clear from context.
Without 〜間, it can be ambiguous; 一年間 is clearer for duration.
Referring to one specific year, such as a calendar year or a year of age.
Used for 'one year' as a unit, like a calendar year or a year in school.
Describing someone or something that is one year old.
Standard counter for age. Note the irregular reading いっさい.
Expressing frequency of once per year.
一年 can mean 'one year' as a point in time or a general count, while 一年間 explicitly marks a duration. Use 一年間 when you want to emphasize 'for a period of one year'.
one year ago
I will be in Japan for one year.
I waited one year.
One year is 365 days.
He is a first-year student.
My daughter turned one year old.
Can be used for animals or inanimate objects, but 一歳 is standard for people.
This dog is one year old.
I get a health checkup once a year.
More formal/literary alternative. 年 (ねん) means 'year' and 一度 (いちど) means 'once'.
A festival is held once a year.
studied for one year
She is one year old.