Why does 在 turn up in 全班同学在老师公布考试时间之后都紧张了起来?

Question

I have two sentences.

1. 总经理（在）假期结束之后立刻开始了紧张的工作.
2. 全班同学在老师公布考试时间之后都紧张了起来.

The first sentence can eliminate 在. About the reason why 在 turns up, in my opinion, it can be the basic structure:

Subject + When + Where + How + Verb + Object

because 老师公布考试时间 is a sentence, adding "之后" just make it indicate the time (老师公布考试时间之后). So between Subject and Time, we need a preposition. However, I cannot explain the grammar structure of the first sentence (lmao).

2 sentences may not follow the basic structure (and I misunderstood). In case I am correct:

1/ why can the first sentence be eliminated 在？

2/ If I am wrong, is there any other structures that match both sentences?

Answer 1

1. 总经理在假期结束之后立刻开始了紧张的工作.

2. 全班同学在老师公布考试时间之后都紧张了起来.

I don't think we should omit 在 in the first sentence either. 在... 之后 is a prepositional phrase indicating when something happens.

在 can be droppable when you put the prep phrase at the beginning of the sentence. E. g.

(在)假期结束之后, 总经理立刻开始了紧张的工作.

(在)老师公布考试时间之后, 全班同学都紧张了起来.

There isn't any special structure involved here. This is all about how/where you want to insert a prepositional phrase in a sentence.

Answer 2

在 in the first sentence tend to be omitted because there is no other kind of people do other activity. In this sentence only one kind of person called 總經理. So it is clear that 總經理[at the particular time][do particular activity]. So even if you omit 在 the sentence is clear enough to understand.

在 in the second sentence have less tendency to be omitted because there are other kind of people do other activity. 全班同學[at articular time][do particular activity(being nervous)] and other kind of person 老師[do other activity(announce time of the test)]. There are two kind of people do things in the sentence so there is less tendency to omit 在.

I think there is more tendency to omit 在 if I modify the sentence and the sentence become 全班同學考試時間公布之後都緊張了起來. In the modified sentence I remove 老師 so there is only one kind of person do thing. And it can have more tendency to omit 在.