The evening conversation, when they were all assembled, had lost much of its animation, and almost all its sense, by the absence of Jane and Elizabeth.

1. evening是否有作定语的用法？没有查到，所以请教老师。

答：evening 可以放在名词前作定语的。如：

I picked up reading in the evening school. 我是在夜校学会看书的。

All the evening dresses were beautiful. 所有的女装晚礼服都很漂亮。

The evening performance will begin at eight o’clock. 夜场演出将于八点开始。

The garden looked lovely in the evening sunlight. 在傍晚的阳光下花园显得十分秀丽。

2. by取意“由于…”还是“在…期间”？我怎么看着两个都可以。

答：by 表原因，意思是“由于”。又如：

He took my pen by mistake. 他错拿了我的钢笔。

He always drank too much and lost jobs by it. 他老是饮酒过量,因此多次失业。

Whether by design or accident his timing was perfect. 不管是计划好的还是巧合，他对时机的把握堪称完美。

3. by前面逗号的作用。

答：全句的结构比较松散，严格说来，句子中至少有两个逗号（最后两个）是可以不用的：

The evening conversation, when they were all assembled, had lost much of its animation and almost all its sense by the absence of Jane and Elizabeth.

至于 when they were all assembled 前后的逗号是为隔开这个状语从句。其实，如果把它放到句首，就只需一个逗号了，如：

When they were all assembled, the evening conversation had lost much of its animation and almost all its sense by the absence of Jane and Elizabeth.

总结一下：作者在 by 前面加逗号，主要是为了避免读者将 and almost all its sense by the absence of Jane and Elizabeth 视为一个单独的整体，因为这个 by 短语 by the absence of Jane and Elizabeth 其实是控制整个谓语（had lost much of its animation and almost all its sense）的。