Question

When Mary came to the river, the horse was standing there, head hung as if abashed.

为什么if 后面可以省略it was ? 这是什么语法规则？

Answer 1

as if the horse/it was abashed
当主句的主语和从句的主语一致且从句的谓语动词的一部分是be时，可以把从句的主语和be同时省略。
一般常用的时间状语从句，条件状语从句，让步状语从句等。
While crossing the street, you must be careful.
Although (we are) tired, we go on working.
If (I am ) invited, i will go there.

Answer 2

因为前面已经有主语the horse, 所以为了语言的减省，as if 后面的从句就省去了这个主语，这里要注意，当省去主语时，was 也要同时省去, 就留下了abashed. 这么说清楚吗？