They will then vote on whether to ask for an extension past March 29th, which is when Britain is due to leave the EU.
这个定语从句本来是可以直接用when从句的。但这样有可能when从句被误读为时间状语从句,修饰谓语动词will vote. 改用which is when...之后,则避免了误读的可能,从句只能是定语从句,修饰March 29th. 这个正是作者的良苦用心所在。
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They will then vote on whether to ask for an extension past March 29th, which is when Britain is due to leave the EU.
他们将投票决定是否要求延长3月29日以后的期限,这个日期也就是英国即将离开欧盟的时候。
(1)which定语从句里面含有一个when引导表语从句,which为非限制性定语从句修饰限定“March 29th”
(2)which代替先行词March 29th并在非限制性定语从句作主语成分.
(3)融合性连接副词when=the time when=the time in which
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