YOU SAID:

He's as dumb as a doornail

INTO JAPANESE

彼はドアネイルのように愚かだ

BACK INTO ENGLISH

He's as stupid as door nails.

INTO JAPANESE

彼はドアの釘のように愚かだ。

BACK INTO ENGLISH

He is as stupid as a nail on the door.

INTO JAPANESE

彼はドアの釘のように愚かです。

BACK INTO ENGLISH

He is as stupid as a nail on the door.