1.
how many years
Used to ask about a duration or quantity of years.
日本に何年住んでいますか。
How many years have you lived in Japan?
この問題は何年も続いているが、まだ解決していない。
This problem has continued for many years, but it still hasn't been resolved.
何年も means 'for many years' (emphatic, not a question).
2.
what year
Used to ask which year something occurred or will occur.
何年に生まれましたか。
What year were you born?
何年 combines 何 (what/how many) with 年 (year).
MEANINGS:
- As a question: how many years / what year
- With も: 何年も = for many years (emphatic, not a question)
COMMON PATTERNS:
- 何年ですか: What year is it?
- 何年に: in what year
- 何年も: for many years
- 何年も前に: many years ago
- 何年生: what school year
RELATED:
- 何か月: how many months
- 何日: how many days / what date