Lz原句写差了.
It is for us,the living,rather,to be dedicated here to the unfinished work which they who fought here have thus far so nobly advanced.
这样看：It is for us(the living,rather,) to be dedicated here to the unfinished work.
对于我们（活着的人）来说,应该献身于那些为完成的工作.
which they who fought here have thus far so nobly advanced.做work的定语从句,修饰work,哪些工作呢?他们（谁呢?who fought here那些在此奋斗过的人们）曾经奋勇推动的未完成的工作.