好像你在问as后面必须跟成分吗?是的,它后面必须跟成分,无论它是用作介词还是连词.
1.作介词用时,后面可以跟名词也可以跟其他的结构:
He is considered as the best student in our class.
He works as a teacher in the school.
He is regarded as clever.
2.做连词用：He is as tall as Tom.
Tall as he is ,he can't reach the picture.