Question

什么时候用to do什么时候用 to be done,

Answer 1

正确的应当这么说：

A sofa is comfortable (for us) to sit in/ on.

Polluted water is to fit (for) people/us to drink.

这种结构中的主语 sofa 既是句子的语法主语也是宾语.

e.g.The music is pleasant (for us ) to listen (to).

省去的 to do 的逻辑主语是人.