YOU SAID:

I could swear I've seen that before.

INTO JAPANESE

私はそれまでを見たことを誓うこと。

BACK INTO ENGLISH

I up it to swear that I saw.

INTO JAPANESE

私はそれを見たことを誓う。

BACK INTO ENGLISH

I swear that I saw it.

INTO JAPANESE

私は私がそれを見たことを誓います。

BACK INTO ENGLISH

I swear I've seen it.

INTO JAPANESE

私はそれを見た誓います。

BACK INTO ENGLISH

I looked at it I swear.

INTO JAPANESE

私は誓うそれを見た。

BACK INTO ENGLISH

I swear it.

INTO JAPANESE

誓います。

BACK INTO ENGLISH

I swear.

INTO JAPANESE

私が保証します。

BACK INTO ENGLISH

I swear.