That river is very dangerous to swim in.

• That river is very dangerous to swim in. (在那條河裡游泳很危險) (正)
• That river is very dangerous to swim. (誤)

解說：不定詞經常放在敘述形容詞之後，用來表示該敘述形容詞所表述的情狀的原因。當這個不定詞為不及物動詞時，其後的介詞不可缺少，如本句中的 in，意指「在那條河裡」(in that river)。