/>before
before作介词时,意思为“在……之前”
如: Annie was born a few weeks before Christmas.
安妮出生在圣诞节前几周.
before作连词时,意思为“在(他人做某事)之前”
如:Before I could catch up with the ball, Tom had beaten him to it.
我还没来得及追上球,Tom就已经抢先一步触到球了.
as
as 作介词时,意思为“作为,以……的身份”
如:I had natural ability as a footballer.
我天生就是当足球运动员的料.
as作连词时,意思为“像…一样” , 多用与构成"as...as..."结构,其中第一个as是副词,后面接形容词或者副词;第二个as是连词,后面接句子
如:I don't think he was ever as fit as he should have been.
我认为他本该更健康的.
当然as作连词还可理解为“当…的时候,在…的同时”
如:The play started as I got there.
我到那里的时候演出开始了.
综上:一般情况下,遇到某个词既是介词又是连词的时候,后面接句子的时候是连词,接短语的是介词.