新概念III课文的一错误?

jk1688

lesson 60 Too early and too late
the 14th line:
The only thing to do was ask them to come half an hour later than the
other guest.

很明显，was后要用非谓语动词，是不能直接用ask的
各位有其它解释么？
谢谢

tasako

俺从网上搜到的

http://bbs.putclub.com/index.php?showtopic=93165

“是一回事。作表语的不定式，如果主语部分中有个do的话，to可以省略。记作“有do无to" 类似的还有：
She could do nothing but cry.
She had no choice but to cry. ”

tasako

我记得老师以前讲过的 ，这个解释应该是对的