least 在虚拟时态中的用法

对不起，你的问题有误，不是least而是lest

lest[所连接的状语从句里常用should/might或原形动词]以免; 生怕; 唯恐 例如：
I obeyed her lest she should be angry.
我得顺着她，免得她生气。
Be careful lest you fall from that tree.
要当心, 以免从树上摔下来。

[用于fear, worry等词之后起连接作用, 无实际意义=that]
I was afraid lest he might come too late.
我怕他来得太晚。

lest 意为以防，以免，从句要用：(should)+动词原形，虚拟。

least用法等同于IN CASE