predicative clauses (表语从句)

you can't eat trees 在句中作表语,叫做表语从句。在它的前面省略了从属连词 that。在口语中,连词 that 是常常省略的[参见 that(conj.) $^{8}$ , noun clauses]:

The truth is he was very shy. 事实是他很腼腆。

此外，wh-词也常常引导表语从句：

The difficulty was how he could prove his ideas to other scientists. 困难在于怎样才能向其他科学家证实他的想法。