YOU SAID:

I seriously do not understand a word you say.

INTO JAPANESE

私はあなたが言う言葉を真剣に理解していません。

BACK INTO ENGLISH

I do not really understand the words you say.

INTO JAPANESE

私はあなたが言う言葉を本当に理解していません。

BACK INTO ENGLISH

I do not really understand the words you say.