YOU SAID:

I saw a bird fly by a mountain

INTO JAPANESE

私は鳥が山のそばを飛ぶのを見た

BACK INTO ENGLISH

I saw a bird fly by the mountain.

INTO JAPANESE

私は鳥が山のそばを飛んでいるのを見た。

BACK INTO ENGLISH

I saw a bird flying by the mountain.

INTO JAPANESE

私は山のそばを飛んでいる鳥を見た。

BACK INTO ENGLISH

I saw a bird flying by the mountain.