The headmaster said that the teaching building ( ）(完工）before May.

would be finished
希望对你有所帮助,有不会的可以再问,我差不多也是这么写的，但答案是wouldhavebeen completed,请问为什么？可能是因为before的关系吧一般情况下,有before这词,都要用完成时过去将来完成时+被动语态哪还有哪些词像before这样接完成时呢？justsince,for,over the years,in the past...years