什么时候用compared 什么时候用compare?

什么时候用compared 什么时候用compare?
compare A to B。把A比作B 。compared to B。与B比起来。 两句都是有比较的意思，为什么第一句用了原形而第二句用了过去式？

被动语态 compared 前应该有be吧
一般句子前都有be动词的 用作短语时 就省略了前面的be
不加时做状语：Compared with Tom.I still have a long way to go .与汤姆比,我还差得远呢.加时做谓语：Teachers are often compared to candles.老师常常被比作蜡烛.
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