Question

3.    Isn't it rude for him to talk to his mother like that?

Answer 1

3. for改为of 本句型中it为形式主语，真正的主语为不定式短语，不定式的逻辑主语常用介词for或of引出。形容词rude和him之间有内在联系，这里应用介词of。