YOU SAID:

dave watched his friends stumble down the stairs

INTO JAPANESE

デイブは友達が階段を降りるのを見ていた

BACK INTO ENGLISH

Dave watched his friend go down the stairs.

INTO JAPANESE

デイブは友人が階段を降りるのを見ていた。

BACK INTO ENGLISH

Dave was watching a friend go down the stairs.

INTO JAPANESE

デイブは友人が階段を降りるのを見ていました。

BACK INTO ENGLISH

Dave was watching a friend go down the stairs.