为什么是：We have planted ten tress and there are twenty more to be planted

sth.to do 是要干某事,这件事是由主语来做的,而sth.to be done是有事要做,这件事本质上不是由主语来做的．
比方说：I have my dinner to cook.这里的饭是我做的,而I have my hair to be cut.头发不是我来剪的,是理发师,但我要剪头发.
你这里的to be planted 的主语是there,所以应该用to be done.