在“it is first/second...time that...。”或在“it/this/that is the
在“it is first/second...time that...。”或在“it/this/that is the +最高级+名词+that...。”结构中,that从句中的动词常用现在完成时;在“it + be...+since从句”这一结构中,be可用一般现在时或现在完成时,since从句一般用过去时。have/has/had +been +动词的现在进行时。为什么这段语法解释看不懂啊?