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.