在由lest , for fear that , in order that , so that , in case of 等引导的目的状语从句中需用虚拟语气。表示以防，以免，等意思。其谓语动词多由should / could / might + 动词原形构成，should也可省略。
例如：①He was punished lest he should make the same mistake again. 他被处以惩罚以免他再犯.
②I reminded her twice of it lest she should forget. 我提醒她两次，怕她忘记 。
③I will not make a noise for fear that I should disturb him. 我不会出声的，以免打扰他。
④I have come all the way here in order that you should understand me. 我到这里来为的是你能理解我 。
⑤He put his coat over his son in case he should catch cold. 他把大衣盖在儿子身上以免他着凉 。
注意：1.如果 in order that 和 so that 前的主句是现在时，其后的从句有时也可用 can , may + 动词原形。
例如：Ask him to hurry up with the letters so that I can sign them. 让他带着信赶快来以便我签字。
 2.在以lest 和in case 引导的目的状语从句中谓语动词若属于过去时，一般要用虚拟语气，should + 动词原形，若谓语动词不是过去时，也可以用成述语气。
例如：take warm clothes in case the weather is cold . 带上厚衣服以防天变冷。