首先更正一下,先行词是不能省略的,哪怕先行词是that也不能省.
先行词如果由that或which引导定语从句,如果that或which在从句中做宾语的话,无论是口语还是书面语都可以省略.
如：This is the TV set that my father bought last year.这里的that是可以省略的.