prior在句子中是什么词性

Prior to crossing roads, the school bus goes first.

老师，prior to crossing roads. prior 是形容词；怎么样理解句子成分？

Prior to crossing roads, the school bus goes first.

【翻译】穿过马路，校车优先。

prior to 是 adj./adv. + pre.（形容词/副词 + 介词）结构，可以理解为“复合介词”。又如：

contrary to

second to

along with

out of

instead of

according to

due to

regardless of