kaigisitu ga aitara, soko de hanasimasyou.

Because 空く is an intransitive verb, and が marks the thing that becomes empty / becomes available.

So:

• 会議室が空く = the meeting room becomes available
• not 会議室を空く ✘

Using が also feels natural because the room is the thing whose state changes.
You could sometimes see 会議室は空いたら in a different context, but が is the most neutral and natural choice here.

空いたら is:

• 空いた = past short form of 空く
• ら = conditional if / when

Together, 空いたら means if it becomes available or when it becomes available.

A very important point: with 〜たら, Japanese uses the past form before ら even when the meaning is about the future.

So:

• 会議室が空いたら = when/if the meeting room becomes free

It does not mean the room already became free in the past.

Because 空く and 空ける are a pair:

• 空く（あく） = to become empty / become available
  
  • intransitive
• 空ける（あける） = to empty something / make something available
  
  • transitive

In this sentence, the room is becoming available by itself from the speaker’s point of view, so 空く is the right verb.

• 会議室が空いたら = when the meeting room opens up / becomes free
• 会議室を空けたら would mean something more like when someone empties the meeting room

It can suggest either, depending on context.

• if = if the room becomes available
• when = when the room becomes available

In this sentence, English often prefers when because it sounds like the speakers expect the room to become free and plan to talk at that time.

So the nuance is often:

• Once the meeting room is free, let’s talk there.

Japanese 〜たら is flexible and often covers both ideas.

そこ refers to the meeting room just mentioned.

Japanese often uses words like:

• ここ = here
• そこ = there
• あそこ = over there

But these words are not only about physical distance. They can also refer back to something already mentioned in the conversation.

So here そこ basically means:

• that place
• in context: the meeting room

That is why the sentence does not need to repeat 会議室 again.

Because で marks the place where an action happens.

Here, the action is 話す = to talk, so the place of that action is marked with で:

• そこで話す = talk there

Compare:

• で = where an action takes place
• に = destination / existence point / specific target in many cases

So:

• そこで話しましょう = let’s talk there ✔
• そこに話しましょう ✘ sounds unnatural here

話しましょう is the polite volitional form of 話す.

It means:

• let’s talk
• shall we talk

It is a polite suggestion that includes the speaker and the listener.

So the tone is cooperative and inviting, not forceful.

Compare:

• 話す = talk
• 話します = will talk / talk (polite)
• 話しましょう = let’s talk

Because Japanese often leaves out subjects when they are obvious from context.

In English, we usually need to say:

• we’ll talk there
• let’s talk there

In Japanese, if the speaker and listener are clearly the people involved, we is simply understood.

So 話しましょう naturally implies:

• let’s talk
• you and I / we should talk

This kind of omission is extremely common in Japanese.

Because 話す can be used without stating the topic when the topic is already understood or not important.

Here the sentence focuses on where and when the talking will happen, not what will be discussed.

If you wanted to specify the topic, you could say something like:

• そこでその件について話しましょう。
  = Let’s talk about that matter there.

But in the original sentence, leaving the topic unstated sounds perfectly natural.

Yes, but the nuance changes a little.

• 空いたら often feels tied to a specific event or sequence:
  when it becomes available, then let’s talk there
• 空けば is also conditional, but can sound a bit more general or hypothetical

In this sentence, 空いたら is very natural because it suggests:

1. the room becomes free
2. then we talk there

That step-by-step feeling fits the situation well.

The comma helps separate the condition from the main action:

• 会議室が空いたら、 = when the meeting room is free,
• そこで話しましょう。 = let’s talk there.

It makes the sentence easier to read.

The comma is helpful and common, but in casual writing it is not always absolutely required.

No. Standard Japanese is normally written without spaces between words:

• 会議室が空いたら、そこで話しましょう。

The spaces are often added in learning materials to make the structure easier to see.

So the natural normal written form is:

会議室が空いたら、そこで話しましょう。