she was not only the largest ship that had ever been built, but was regarded as unsinkable.
翻译为,这船不仅是造船史上建造的最大的一艘船,而且也被认为是不会沉没的,
请问,that had ever been built在这是什么意思,启什么作用,我认为不是过去曾经造吗,怎么翻译在造船史上,