Ten houses had been built by them by the end of last year这是过去完成时态吗？对，确切的说时过去完成时态的被动语态。had后面不加not吗？原句是肯定句，转换成被动语态后仍然是肯定句那改成问句呢？Had they built ten houses by the end of last year?had提前就行原句中有already是不是就变否定句？不是啊，already只是一个程度副词，没有否定含义。