怎样判断一个句子是否是定语从句?

定语从句是形容词性从句,是用来修饰句子的,即在句中做定语,修饰一个名词或代词,被修饰的名词,词组或代词即先行词.定语从句通常出现在先行词之后,由关系词（关系代词或关系副词）引出.举个例子：He is the man （whom/ that I saw yesterday）.他就是我昨天见的那个人.（whom/that在从句中作宾语.括号内容是用来修饰the man.,即相当于一个形容词.而名词性从句 分为主语从句、宾语从句、表语从句和同位语从句,下面简单举例 eg:（Who will win the match） is still unknown.由who引导主语从句,作主语 She will give （whoever needs help a warm support）由whoever引导宾语从句,作句子宾语 That is （why he didn’t come to the meeting）由why引导表语从句,在句子中作表语 I have no idea （when he will come back home.）由when引导同位语从句,在句子中作同位语,对idea作具体解释.