先行詞并不能決定that能不能省略,要看關(guān)系代詞that本身在從句中的作的成分.
1.先行詞為人：
1）人做主語－－－關(guān)系代詞用who且不可省略
2）人做賓語－－－關(guān)系代詞用whom／who/不填
2.先行詞為物：
1）物做主語－－－關(guān)系代詞用that/which
2）物做賓語－－－關(guān)系代詞用that/which/不填
不填就是可以省略.
舉個例子：A)The boys that are playing footballl are from America.
that修飾限定男孩們,且男孩們在句中做主語.
省略掉之后就變成：The boys are playing football are from America.很顯然是不可以的
B)The news (that) she heard is true.
that修飾限定消息,消息又在主句中做主語,但是that在從句中作賓語成分,所以可以省去.
另外,指人可用that who,以下情況多用who;Those people做先行,There be的結(jié)構(gòu)中；先行指人不定代,從中做主who要在；
例：People who create computer viruses are called hackers.
一般多用who,但并不代表就不能使用that,這僅是約定俗成的用法.