undersea mountains generally form when volcanic mountains rise up from the seafloor but fail to reach the surface,while (those that) break the surface become islands. 这里that有什么意义?请详解!谢谢~ 定语从句that which一般情况可以省略吗?什么时候不能省略?谢谢!~
undersea mountains(主语) generally form(谓语) when volcanic mountains rise up from the seafloor but fail to reach the surface时间状语 while (those《those 指代volcanic mountains 》 that break the surface become islands.与上面的句子是并列 that是从句的主语 连接break the surface作定语修饰those,也就是火山; 然而冲破海平面的火山就成为了岛。