になったの

became

→ に
phrase Uncommon #15,511
became The past form of になる (“to become”) followed by の, which can add an explanatory tone or soften a question. The の is often left untranslated.
JA: いつから先生になったの?
EN: When did you become a teacher?

昏睡状態になったの

I went into a coma.

— グッバイ、レーニン! (2003)

心が石になったの

My heart has turned to stone.

— ウィッチ (2016)

どうして、そんなことになったの?

How did it come to this?

トム、何歳になったの?

Tom, how old are you now?

なぜ教師になったの?

Why did you become a teacher?

生理になったの?

Did you get your period?

— 箪笥<たんす> (2003)

今難しい事態になったの

Things have gotten difficult now.

— オデッセイ (2015)

人間になったの

Did you become human?

— ブゴニア (2025)

それでお金が必要になったの

That's why I needed money.

— グッバイ、レーニン! (2003)

話してこの間すごい夕立になったの

Speaking of which, we had a really heavy evening rainstorm the other day.

— ふたりのベロニカ (1991)

どうしてあなたは一人になったの?

How did you end up alone?

どうして警察官になったの?

Why did you become a police officer?

トムはどうやって億万長者になったの?

How did Tom become a billionaire?