YOU SAID:
a mountain is eighty years old
INTO JAPANESE
山は80歳
BACK INTO ENGLISH
The mountain is 80 years old.
INTO JAPANESE
その山は80年の古い。
BACK INTO ENGLISH
The mountain is eighty years old.
INTO JAPANESE
その山は80歳だ。
BACK INTO ENGLISH
The mountain is eighty years old.
That didn't even make that much sense in English.