关于一个动词短语的用法
请问what we should do is to talk about.
what we should do is talking about...这两句中talk about是动词短语,但是用在is后面时,应该这么用呢?
类似的一句还有：this subject is mainly to talk about....请问这种表述是对的吗？还是this subject mainly talk about...或者是this subject is mainly talk about?：）