② This river is dangerous to swim in after the rain.

何故　一文目はIn afternoon なのですか？
→意味の切れ目が違います
to swim in / afternoon です

It is dangerous to swim in this river after the rain.
↓this river を 主語にしてIt の位置に移動
This river is dangerous to swim in after the rain.
と書き換えたと考えてみてください
tough 構文 ということがあります

①について
It is difficult to finish my homework in an hour.
↓
My homework is difficult to finish in an hour.
と書き換えできます

訂正
I think (that) it is difficult to finish my homework in an hour.
→第3文型️⭕️(第5文型❌)

わかりました！丁寧にありがとうございます！