kaigi ga owattara, kaigisitu ga akimasu.

Because the sentence has two separate clauses, and each clause has its own subject.

• 会議が終わったら
  • 会議 = the meeting
  • が marks the meeting as the thing that ends
• 会議室が空きます
  • 会議室 = the meeting room
  • が marks the meeting room as the thing that becomes available / opens up

So the structure is basically:

• When the meeting ends, the meeting room becomes available.

Using が twice is completely normal here.

This is a very common point of confusion.

In Japanese, the pattern verb in past short form + ら makes the conditional 〜たら. It often means:

• if
• when
• after

So 終わったら does not mean the action is already in the past from the speaker's point of view. It means:

• when it has ended
• after it ends
• once it ends

So:

• 終わる = to end
• 終わった = ended
• 終わったら = if/when/after it ends

This is just how the grammar works.

Here, 〜たら is best understood as when or after.

The sentence describes a natural sequence of events:

1. the meeting ends
2. then the meeting room becomes available

So the nuance is not really a hypothetical if. It is more like:

• When the meeting is over, the meeting room will be available.
• After the meeting ends, the meeting room opens up.

In other contexts, 〜たら can also mean a true if, but here it sounds more temporal than hypothetical.

Because 終わる is an intransitive verb, while 終える is transitive.

• 終わる = to end
• 終える = to end something / to finish something

A meeting itself ends, so Japanese naturally says:

• 会議が終わる = the meeting ends

If you used 終える, you would need someone as the subject, because someone is ending something:

• 司会者が会議を終える = The chairperson ends the meeting.

In your sentence, the focus is on the meeting ending by itself, so 終わる is the natural choice.

Because 空きます focuses on the room becoming available, not simply being empty.

• 空きます = will open up / will become vacant / will become available
• 空いています = is open / is vacant / is empty

In this sentence, the idea is:

• once the meeting ends, the room will open up for use

So 空きます fits the change of state very well.

Compare:

• 会議室が空きます。
  = The meeting room will become available.

• 会議室が空いています。
  = The meeting room is available / empty.

Here it is read あきます.

So:

• 空きます = あきます

This verb often means:

• to become empty
• to become available
• to open up

The reading すきます also exists for 空く, but it is more commonly used in situations like:

• a road becoming less crowded
• a train emptying out a bit
• time opening up in your schedule

For a room becoming vacant/available, あきます is the standard reading to learn first.

Sometimes yes, but the nuance changes.

Original:

• 会議が終わったら、会議室が空きます。

This sounds like a straightforward description of what happens.

If you say:

• 会議が終わったら、会議室は空きます。

then は gives 会議室 a topic or contrast nuance, something like:

• As for the meeting room, it will be available
• perhaps implying contrast with something else

And if you changed the first が to は:

• 会議は終わったら...

that would be unnatural here.

So in this sentence, using が for both is the most natural neutral choice.

Japanese often focuses on events and states rather than explicitly naming the person responsible.

This sentence is talking about what naturally happens:

• the meeting ends
• the room becomes available

It does not matter who officially clears the room or manages the schedule. Japanese often leaves that kind of detail unstated when it is not important.

This is very normal. English also sometimes does this:

• The room becomes available after the meeting.

No person needs to be mentioned.

The comma is helpful, but it is not absolutely required.

• 会議が終わったら、会議室が空きます。

The comma shows a pause between:

1. the condition/time clause
2. the main clause

It makes the sentence easier to read.

Without the comma, the sentence would still be understandable:

• 会議が終わったら会議室が空きます。

So the comma is mostly for readability and natural phrasing.

Grammatically, 〜たら can mean if, so that translation is possible at a basic level.

However, in this specific sentence, when/after is usually better because it sounds like a normal expected sequence, not an uncertain condition.

So these are more natural:

• When the meeting ends, the meeting room will be available.
• After the meeting is over, the meeting room will open up.

Using if in English can make it sound more doubtful than the Japanese usually does here.

Yes, you could express a similar idea that way, but the nuance is a little different.

• 会議が終わったら、会議室が空きます。

  • focuses on the moment the meeting ends and what happens next
  • very natural for a schedule/result relationship

• 会議のあとで、会議室が空きます。

  • means after the meeting
  • understandable, but slightly less direct in showing the event sequence

In many everyday situations, 〜たら sounds especially natural because it links the two events tightly:

• once X happens, Y happens

So the original sentence is a very natural way to say it.