YOU SAID:

i dont know hat to write

INTO JAPANESE

私は書くために帽子を知らない

BACK INTO ENGLISH

I don't know the hat to write

INTO JAPANESE

私は書くべき帽子を知らない

BACK INTO ENGLISH

I don't know which hat to write.

INTO JAPANESE

どの帽子を書けばいいのかわかりません。

BACK INTO ENGLISH

I don't know which hat to write.