不是的,可以做插入语,比如to one’s surprise.还有,不定式短语作插入语.
　　能用作插入语的不定式短语有：to be frank,to be honest,to be sure,to tell you the truth,to make matters worse,to sum up,to start with,to begin with等.　如：　To be frank,I don’t quite agree with you.　坦率地说,我不太同意你的意见.　To tell you the truth,I’m not so interested in the matter.　跟你说实话,我对这件事情的兴趣不大.　To sum up,success results from hard work.　总而言之,成功是艰苦努力的结果.