lest的意思是唯恐，以免，免得。它是连词。
1.经典语法书：所连接的状语从句里常用should或原形动词，表示以免; 生怕; 唯恐 ：
They spoke in whispers lest they should be heard.
他们低声耳语惟恐被人听见。
I obeyed her lest she should be angry.
我得顺着她，免得她生气。
He wrote a letter home lest his parents worry.
他写了封信回家，免得父母结记。
I reminded him several times lest he should forget.
我几次提醒他，唯恐他忘了。
She was afraid lest she had revealed too much.
她担心她泄露得太多了。
The president has gone along with the hardliners lest they be tempted to oust him
总统同意了强硬派的意见，以免他们产生赶他下台的念头。
I was afraid to open the door lest he should follow me
我不敢开门，唯恐他还跟着我。
Read in a good light lest it should hurt your eyes.
到明处来看书，免得搞坏了视力。
Close the window quickly lest the raindrops ( should) slant in.
快关窗户，别让雨点潲进来。
She went into the bedroom softly lest she should wake her child.=
She went into the bedroom softly so as not to wake her child.
她轻轻地走进卧室，生怕惊醒了孩子。
考生们应当牢记以上这个规则，貌似各类考试只承认lest从句里主语后接should或原形动词用法。
用在for fear that或lest(in case)引导的从句中，表示“怕…”“万一…”“以免”“免得”等意思。从句的谓语动词用should 动词原形，should 可以省略。如：
She closed the windows for fear that she (should) catch cold. 她关上窗户，以免着凉。
The officer forbade the use of artillery lest the city’s industry be damaged.
军官禁止使用大炮，以免损坏城市的工业。
Lest the wall should collapse, they evacuated from the building.
为了防止墙倒塌，他们从大楼里疏散了。
In case you should need any help, here's my number.
万一你需要帮助的话，这是我的电话号码。
I'll stay around in case you need me.
我就待在这儿，也许你用得着我。
Take some spare clothes in case you get wet.
多带几件衣服，以备身上弄湿了好穿。
She said nothing in case her temper slipped out of gear.
她什么都没说，免得按捺不住情绪。(陈述语气)
You hang around here in case he comes, and I'll go on ahead.
你在这附近等着以防万一他来了，我继续往前走。(陈述语气) 《牛津高阶英汉双解词典》
Leave your key with a neighbour in case you lock yourself out one day.
给邻居留把钥匙，以防有朝一日把自己反锁在外面。
《柯林斯高阶英汉双解学习词典》
I was going to get out of the building in case it went up
我怕大楼起火，正打算要跑出去。(陈述语气) 《柯林斯高阶英汉双解学习词典》
2.实际语言运用中，lest从句使用陈述语气也是可以的。如：
I had to grab the iron rail at my side lest I slipped.
我不得不抓住我这一侧的铁栏杆，以免滑倒。
He wouldn’t put the letter in his pocket lest he forgot about it.
他不把那封信放在口袋里，为的是怕把那封信给遗失了。
