Learn Hardcore Chinese: Wǒmen juédìng míngtiān yìqǐ cānjiā yígè xiǎo jùhuì. - We decided to go to a small get-together together tomorrow.

The time word 明天 (míngtiān, tomorrow) already tells you the action is in the future, so 决定参加 is understood as “decide to attend (in the future)”.

If you really want to emphasize the future, you can add words like:

会: 我们会明天一起参加一个小聚会。
要: 我们要明天一起参加一个小聚会。

But they are not grammatically required; the original sentence is perfectly natural.

Why is 明天 placed after 我们 and before 一起? Could it go somewhere else?

A very common word order in Chinese is:

Subject + Time + Manner + Verb + Object

Here:

Subject: 我们
Time: 明天
Manner: 一起
Verb: 参加
Object: 一个小聚会

So 我们明天一起参加一个小聚会 follows that pattern exactly.

Some other natural options are:

明天我们一起参加一个小聚会。 (Putting the time at the very beginning is also common.)
我们一起明天参加一个小聚会。 (Possible, but less common; most teachers recommend putting the time before 一起.)

You normally wouldn’t put 明天 after the verb here, e.g.
✗ 我们一起参加一个小聚会明天。 (unnatural in standard Mandarin).

What exactly does 一起 do here, and where can it go in the sentence?

一起 (yìqǐ) means “together”. It works like an adverb and usually goes before the main verb:

我们一起参加一个小聚会。
“We attend a small gathering together.”

Typical positions for 一起:

After the subject: 我们一起参加……
After a time word: 我们明天一起参加……

You normally cannot move it to the end like in English:

✗ 我们参加一个小聚会一起。 (wrong / very unnatural)

So, put 一起 before the verb it modifies.

Why use 参加 instead of 去 for “attend / go to” a gathering?

Both 参加 and 去 can involve movement, but they focus on different things:

参加 (cānjiā): “to participate in, to take part in”
Used for events, activities, meetings, parties, competitions, classes, etc.
Emphasis: you are a participant in an event.
- 参加比赛 – take part in a competition
- 参加会议 – attend a meeting
去 (qù): “to go (to a place)”
Emphasis: the movement to a location.
- 去北京 – go to Beijing
- 去他家 – go to his house

A 聚会 is an event, so 参加聚会 = “attend / take part in a gathering”, which is more precise than 去聚会 (“go to a gathering”).

Why do we need 一个 before 小聚会? Can’t we just say 小聚会?

In Chinese, when you talk about one specific countable thing, you usually need:

Number + Measure word (classifier) + Noun

Here:

Number: 一 (one)
Measure word: 个
Noun (with adjective): 小聚会

So 一个小聚会 literally means “one small gathering” and corresponds to English “a small gathering.”

You can sometimes drop 一, 个, or both, but that changes the feel:

小聚会 (by itself) feels more general, like “small gatherings (as a type)” or “the small gathering(s)” depending on context.
一个小聚会 is clearly one specific small gathering.

That’s why 一个小聚会 is the natural choice here.

Why is 一 pronounced yí instead of yī in 一个小聚会?

This is a common tone sandhi rule for 一 (yī):

Before a 4th-tone syllable, 一 changes to 2nd tone (yí).
- 一个 (gè is 4th tone) → yí gè
- 一次 (cì is 4th tone) → yí cì
Before a 1st, 2nd, or 3rd tone, 一 usually changes to 4th tone (yì).
- 一天 (tiān, 1st) → yì tiān
- 一年 (nián, 2nd) → yì nián

So in 一个小聚会, you naturally say:

yí gè xiǎo jùhuì
even though it is written as 一个.

Why is 小 placed before 聚会? Could we say 聚会小?

In Chinese, adjectives normally come before the noun they modify:

小聚会 – a small gathering
大城市 – a big city
新手机 – a new phone

So 小聚会 (small + gathering) is correct.
聚会小 is wrong in this context; that order doesn’t work for describing a noun in standard Mandarin.

Also, 小 here is just descriptive:

You can omit it: 一个聚会 – “a gathering” (no size implied)
Adding 小 gives a nuance of small / informal / cozy depending on context.

What is 决定 here: a verb or a noun? How is it different from 打算 or 要?

In this sentence, 决定 (juédìng) is a verb, meaning “to decide (to do something)”:

我们决定明天一起参加一个小聚会。
“We decide / have decided to attend a small gathering together tomorrow.”

As a noun, 决定 means “decision”:

我们的决定 – our decision
做一个决定 – make a decision

Differences from similar words:

决定 – a firm decision (often after thinking):
我们决定去北京。 – We have decided to go to Beijing.
打算 – plan/intend, often less firm / more casual:
我们打算去北京。 – We plan / intend to go to Beijing.
要 – be going to / want to, often used for strong intention or arranged future:
我们要去北京。 – We are going to Beijing / We really intend to go.

So 决定 emphasizes the act of deciding, and it usually feels more deliberate and definite.

Could we move 明天 to the very beginning, like 明天我们决定一起参加一个小聚会?

Yes, that is grammatically correct and natural:

明天我们决定一起参加一个小聚会。

The focus shifts slightly:

我们决定明天一起参加一个小聚会。
More neutral; typical subject + time order.
明天我们决定一起参加一个小聚会。
Slightly more emphasis on “tomorrow” (e.g. in contrast to another day).

Both are fine in everyday speech.

Should there be a 了 after 决定 to show the decision is already made?

You can add 了, but you don’t have to. The basic sentence:

我们决定明天一起参加一个小聚会。

already sounds like a present fact (“We have decided / we decide…”).
If you really want to emphasize that the decision is already completed, you could say:

我们已经决定了，明天一起参加一个小聚会。
“We’ve already decided: we’ll attend a small gathering together tomorrow.”
我们决定了明天一起参加一个小聚会。
(also acceptable; 了 here marks the completion of “deciding”.)

In many contexts, especially in spoken Chinese, the original sentence without 了 is fully natural and understood as a firm decision about the future.