定语从句有先行词,且先行词在从句中充当成分.例如：She is the girl who is good at maths.她就是数学好的那个女孩.有先行词 the girl ,先行词在从句who is good at maths中由who替代,作从句的主语.who被称为关系代...定语从句中只用that不用which的情况