(1)by night 在夜间,趁黑夜这个环境或者掩护去从事某种特定活动。

The robbers fled by night.  

强盗趁天黑而外逃.

The enemy fled in disorder by night.  

敌人趁黑夜狼狈溃逃.

They marched by night.  

他们夜里进军.

He works in an office by day and drives a taxi by night.
他白天上班, 晚上开出租车。

(2)at night 的意思是 at night 在夜里during any night，通常表泛指。

I often work at night. 

我经常在夜间工作。

He’s afraid to go out alone at night. 

他害怕夜晚独自出去。

It’s becoming dangerous to go out alone at night. 

夜间独自外出已经越来越危险了。

Please ensure that all the lights are switched off at night. 

夜间请务必将所有的灯关掉。

He came home very late at night.
他夜里很晚才回家。
We often work at night.
我们常在夜里工作。
She found him drunk when she came home at night.
她晚上回家时, 经常发现他醉醺醺的。

(3)in the night的意思是 during one particular night，通常表特指。

The storm blew over in the night. 

风暴在夜里停止了。

I had to get up in the night. 

那天夜里我非起来不可。 

The temperature fell sharply in the night. 

那天夜间温度急剧下降。

I’m exhausted. The baby woke up three times in the night. 

我累坏了，孩子夜里醒了三次。

这一含义有时可用 during the night 来表示。

Everything was quiet during the night. 

昨夜万籁俱寂。