SAT语法（2013年一月）,During television's early years,many stations broadcast programming only until midnight,(after which television screens will display )a static image known as a test pattern.括号内为需要修改的内容为什么要改
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SAT语法（2013年一月）,During television's early years,many stations broadcast programming only until midnight,(after which television screens will display )a static image known as a test pattern.括号内为需要修改的内容为什么要改
SAT语法（2013年一月）,
During television's early years,many stations broadcast programming only until midnight,(after which television screens will display )a static image known as a test pattern.
括号内为需要修改的内容
为什么要改成after which television screens displayed
而不是.had displayed
原句中到底哪一个在前发生?
SAT语法（2013年一月）,During television's early years,many stations broadcast programming only until midnight,(after which television screens will display )a static image known as a test pattern.括号内为需要修改的内容为什么要改
先是播放节目到半夜,节目结束后是播放静止画面.
display发生在broadcast之后,不能用表示“过去的过去”的过去完成时,而应该与broadcast一样,用一般过去时（注意,这里的broadcast不是动词原形,而是过去式）.
如果把这句话改成一般现在时，你觉得定语从句要用过去时吗？因为这是表示一个经常发生的事，所以用一般现在时即可。
那么再把它放在过去的环境中，都应该用一般过去时