先行词并不能决定that能不能省略,要看关系代词that本身在从句中的作的成分.
1.先行词为人：
1）人做主语－－－关系代词用who且不可省略
2）人做宾语－－－关系代词用whom／who/不填
2.先行词为物：
1）物做主语－－－关系代词用that/which
2）物做宾语－－－关系代词用that/which/不填
不填就是可以省略.
举个例子：A)The boys that are playing footballl are from America.
that修饰限定男孩们,且男孩们在句中做主语.
省略掉之后就变成：The boys are playing football are from America.很显然是不可以的
B)The news (that) she heard is true.
that修饰限定消息,消息又在主句中做主语,但是that在从句中作宾语成分,所以可以省去.
另外,指人可用that who,以下情况多用who;Those people做先行,There be的结构中；先行指人不定代,从中做主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,这仅是约定俗成的用法.
