The river we swim in the summers is snowmelt coming off of the mountain.

英訳に間違いが有れば正してください。

我々が夏に泳ぐ川は山からの融雪だ。
The river we swim in the summers is snowmelt coming off of the mountain.

答え swim in inにする

The river we swim in in the summers is snowmelt coming off of the mountain.

swimは自動詞でswim in the riverとなる