貌似没见过，不过be to do 是将来做某事的意思，mainly是副词大概主要的意思可修饰be，i guess~
后加doing,即mainly doing sth
Do they mainly live on bamboo?What are your government trying to do? 做这种题目要看句子有没有be动词没有be动词再请助动词does do 然后看主语是否第三人称 是就用does
There is a saying "Practice makes perfect". It means that fi you want to learn something, you should learn it from experience. Only in this way can you understand what it really is. Knowledge from books usually abstrct to learn...