He threw himself from his horse and lay still as if shot. (=as if he had been shot）

括号里的分句是被补齐后的结果。

我认为这不是虚拟语气。as if 从句的主句是he threw himself and lay still，谓语动词是threw 和 lay，我们把主、从句谓语动词发生的早晚，作一比较：threw （掉下）lay （躺）是一般过去时，had been shot（被子弹射中）是过去完成时，说明发生在threw 和 lay 之前，谁早谁晚应该很清楚：shoot 早于 lie。设想一下，“先被击中，才躺下”，还是“先躺下，再被击中”？所以，这不是“虚拟语气”，而是陈述语气。不要因为有 as if 就是虚拟语气，我们换成其他状语从句，也得用过去完成时。例如：

He lay still after he had been shot. （本句有after 表示先后，也可说after he was shot）

He lay still as soon as he had been shot.

He lay still because he had been shot.