课本上的例句:1.Designing clothes is not a man's job.2.Being a flight attendant sounds exciting.3.Writing a gossip column could be fun.4.Directing a TV show would be interesting.为什么前面的第一个动词都加ing?直接用原型不行吗?麻烦英语好的朋友帮忙解释下,我初学英语,谢谢了