YOU SAID:

My zipper is loose and it fell on my lap

INTO JAPANESE

私のジッパーが緩んでいる、それは私の膝に落ちた

BACK INTO ENGLISH

My zipper is loose and it fell down on my knees

INTO JAPANESE

私のジッパーが緩んでいる、それは私の膝の上に倒れて

BACK INTO ENGLISH

My zipper is loose and it fell on my knees