Learn Hardcore Korean: hoeuisire deureogajamaja joyonghaeyo. - The meeting room is quiet as soon as you enter.

-자마자 attaches to the verb stem to mean as soon as…, indicating that the action in the main clause follows immediately after the action in the subordinate clause.

Here 들어가자마자 = 들어가(‘to enter’) + -자마자 = as soon as you enter.
Thus 회의실에 들어가자마자 조용해요 means It’s quiet the moment you enter the meeting room.

Why is there no subject in 회의실에 들어가자마자 조용해요?

Korean often omits the subject when it’s clear from context. In this sentence:

The unspoken subject of 조용해요 is understood to be the meeting room (or the atmosphere inside it).
You could say 회의실이 들어가자마자 조용해요, but Koreans typically drop 회의실이 because it’s obvious what’s being described.

Why is 회의실 marked with 에 instead of 에서?

With verbs of motion like 들어가다 (“to enter”), -에 marks the destination:

회의실에 들어가다 = “to go/enter into the meeting room.”
에서 would mark the location where an action takes place (e.g. 회의실에서 회의하다 “to hold a meeting in the conference room”).

Why is 조용하다 conjugated as 조용해요 here?

조용하다 is the dictionary form (“to be quiet”). To make it polite present tense you:

Drop -다 → 조용하-
Add -어요 to the 하- stem → 조용해요
So 조용해요 means is quiet.

Does 조용해요 refer to people or the room being quiet?

It refers to the room (or its atmosphere) becoming quiet. Korean regularly omits subjects when they’re obvious. The implication is the moment we/you enter, the meeting room is quiet.

Could I say 회의실에 들어서자마자 instead of 들어가자마자?

Yes. 들어서다 means “to step into/enter,” so 들어서자마자 + -자마자 = as soon as (one) steps in. Both are correct, though 들어가다 is more common for “enter.”

How would the sentence change if I wanted the past tense (“It became quiet as soon as we entered”)?

You have two natural options:

회의실에 들어가자마자 조용해졌어요 (using 조용해지다 “to become quiet” in past polite form)
회의실에 들어가자마자 조용했어요 (simple past of 조용하다); this is also acceptable but 조용해지다 emphasizes the change of state.

What’s the difference between -자마자 and -면 (as in 들어가면 조용해요)?

-자마자 = as soon as…, stresses immediate succession with no gap.
-면 = if/when…, more general or hypothetical, doesn’t emphasize instant timing.
• 들어가자마자 조용해요 → “It’s quiet the moment you enter.”
• 들어가면 조용해요 → “If/when you enter, it’s quiet,” more like a general fact.