1.
one year; a year
A period of one year.
一年は十二か月ある。
There are twelve months in a year.
日本に来て一年になる。
It's been one year since I came to Japan.
一年で一番寒い月は一月だと言われている。
It is said that the coldest month of the year is January.
2.
first year; first grade
The first year of school or the first grade. Often used as 一年生.
息子は小学校一年生です。
My son is in first grade of elementary school.
大学一年のとき、日本語の勉強を始めた。
I started studying Japanese in my first year of university.
一年 means "one year" (duration) or "first year" (ordinal).
COMMON EXPRESSIONS:
- 一年中: all year round
- 一年間: for one year (emphasizes duration)
- 一年生: first-year student, first grader
- 一年に一度: once a year
NOTE ON COUNTING YEARS:
- 一年 (1 year)
- 二年 (2 years)
- 三年 (3 years)
- 四年 (4 years) — not しねん
- 五年 (5 years)