连词是虚词,它不能独立担任句子的成分,它主要起的是连接词与词之间的作用.连词有：and,but,or,nor,so,therefore,yet,however,for,hence,as well as,both...and...,not only...but also,either...or,neither...nor...,then等．
介词又叫前置词,就是放在名词或相当于名词的前面．常见的介词有：about,after,at,for,from,in,of,on,to
就用for来举例子吧,它即可以出现在连词中,又可以出现在介词中.那区别是什么呢：
e.g.He is absent today,for he is ill.他今天没有来,因为他病了.(for是并列连词,不能置于含两个并列分句的句子的句首,只能将其放在两个分句的中间．)
e.g.Her illness accounts for his absence.他因病缺席．(for表原因,目的等．可直接放置于名词之前．absence是名词．)
