xìngkuī wǒmen xiān jùcān le, bùrán dàjiā huì hěn è.

幸亏 means something like fortunately, luckily, or more literally it’s a good thing that... It often introduces a situation that prevented a bad result.

In this sentence:

幸亏我们先聚餐了，不然大家会很饿。
= Luckily, we ate together first; otherwise everyone would have been very hungry.

So 幸亏 highlights that something bad was avoided.

Compared with 还好:

• 幸亏 strongly suggests a bad outcome was narrowly avoided
• 还好 often just means fortunately / it’s not too bad / good thing

For example:

• 幸亏我带了伞，不然就淋湿了。
  Good thing I brought an umbrella, otherwise I would have gotten soaked.

• 还好我带了伞。
  Luckily I brought an umbrella.

Both can sometimes work, but 幸亏 more clearly sets up the otherwise... idea.

先 means first or beforehand, and in Chinese it usually goes before the verb it modifies.

So:

• 先聚餐 = eat together first / have the meal first

This follows a very common pattern:

• 先 + verb
• 先吃
• 先看
• 先说
• 先去

Examples:

• 我们先吃饭。
  Let’s eat first.
• 你先说。
  You speak first.

So 我们先聚餐了 means we had the group meal first.

Not exactly.

聚餐 means to gather for a meal or to dine together as a group. It has a social/group feeling.

吃饭 is the more general eat / have a meal.

So:

• 吃饭 = eat, have a meal
• 聚餐 = have a group meal, get together to eat

In this sentence, 聚餐 works well because 大家 appears later, so the idea is that people gathered and ate together.

You could think of 聚餐 as something like:

• a group dinner
• eating together
• a get-together meal

It is a bit more specific and social than plain 吃饭.

Here 了 marks that the action has been completed or that a new situation has occurred.

So:

• 我们先聚餐了 = we ate together first / we already had the meal first

Without 了, the sentence could sound more like a general statement, a plan, or something less clearly completed.

In this sentence, the speaker is talking about something that already happened and that prevented another result:

• 幸亏我们先聚餐了，不然大家会很饿。

That is why 了 is natural here: the meal happened already.

不然 means otherwise or if not.

It introduces the result that would happen if the first part were not true.

So the structure is:

• 幸亏...，不然...
• Good thing..., otherwise...

In this sentence:

• 幸亏我们先聚餐了
  Good thing we ate together first
• 不然大家会很饿
  otherwise everyone would be very hungry

A very similar word is 要不然, which also means otherwise and is often interchangeable in everyday speech.

Here 会 does not just mean simple future. It often means would, will likely, or can be expected to depending on context.

Because 不然 sets up a hypothetical result, 会 is best understood as would:

• 不然大家会很饿。
  Otherwise everyone would be very hungry.

So this is not just a neutral future statement. It is a result that would happen under that condition.

Compare:

• 大家很饿。
  Everyone is hungry.
• 大家会很饿。
  Everyone will be / would be hungry.

In Chinese, when an adjective is used as a predicate, 很 is often inserted naturally, even when it does not mean a strong very in English.

So:

• 他很忙。
• 我很累。
• 大家很饿。

These often mean simply:

• He’s busy
• I’m tired
• Everyone is hungry

not necessarily very busy/tired/hungry.

In this sentence, 会很饿 is the normal, natural phrasing.

You can say 会饿, but it may sound more like will get hungry or be slightly less neutral depending on context. 会很饿 makes the state sound clearer and more natural here.

大家 means everyone or all the people involved.

The speaker first says:

• 我们先聚餐了
  we ate together first

Then:

• 不然大家会很饿
  otherwise everyone would be hungry

Using 大家 broadens the focus from just we as actors in the first clause to everyone present / everyone in the group in the result clause.

In many situations, 我们 and 大家 might refer to the same group, but 大家 sounds more natural when talking about the effect on the whole group.

It probably does, but not always automatically.

In Chinese, 我们 can be:

• inclusive: the speaker + listener + maybe others
• exclusive: the speaker + others, but not the listener

Chinese does not always clearly mark that distinction.

In this sentence, context decides. Since the sentence is about a shared meal and 大家, it often sounds like the speaker is talking about a group that likely includes the listener, but grammar alone does not force that interpretation.

It is mainly talking about a real past event with a hypothetical bad result that did not happen.

Breakdown:

• 我们先聚餐了 = the meal already happened
• 不然大家会很饿 = if that had not happened, everyone would be hungry

So the speaker is saying:

• the real event: we ate first
• the unreal result: otherwise everyone would have been hungry

This is a very common Chinese way to express a bad outcome that was avoided.

Yes. 幸亏 often appears with 不然, but it does not have to.

For example:

• 幸亏你提醒我。
  Good thing you reminded me.
• 幸亏我没迟到。
  Luckily I wasn’t late.

When 不然 is added, the sentence explicitly states the avoided result:

• 幸亏你提醒我，不然我就忘了。
  Good thing you reminded me, otherwise I would have forgotten.

So in your sentence, 不然 makes the contrast more complete and vivid.

Yes, you could say:

幸亏我们先吃饭了，不然大家会很饿。

That would be completely understandable and natural.

The difference is:

• 吃饭 = more general
• 聚餐 = more specifically eat together as a group

So 聚餐 fits especially well if the focus is on a shared meal or group gathering.

If you just want simple everyday Chinese, 吃饭 is often the easier and more common choice.

A useful pattern is:

幸亏 + clause, 不然 + clause

Meaning:

Good thing that..., otherwise...

Your sentence follows it exactly:

• 幸亏我们先聚餐了，
  Good thing we ate together first,
• 不然大家会很饿。
  otherwise everyone would be hungry.

You can reuse this pattern in many situations:

• 幸亏我早出门了，不然就迟到了。
  Good thing I left early, otherwise I would have been late.

• 幸亏他带了地图，不然我们会迷路。
  Good thing he brought a map, otherwise we would get lost.

This is a very practical pattern for everyday Mandarin.