Learn Hardcore Chinese: xiàgè xīngqī wǒmen juédìng yìqǐ qù gōngyuán. - Next week we have decided to go to the park together.

In Chinese, time expressions very often come at the beginning of the sentence, before the subject, or right after the subject. All of these are possible and grammatical:

下个星期我们决定一起去公园。
我们下个星期决定一起去公园。
我们决定下个星期一起去公园。

They all mean essentially the same thing. Putting 下个星期 first:

sets the time frame right away
is very natural and common in spoken and written Chinese

Putting the time at the very end (我们决定一起去公园下个星期) is possible but sounds awkward in this type of sentence and is rarely used. Think of Chinese as generally following:

(Time) + Subject + (Time) + Manner + Verb + Object

with time phrases often at or near the front.

Why is there a 个 between 下 and 星期? Can I just say 下星期?

个 here is a measure word (classifier). Literally, 下个星期 is next + (measure) + week, similar to next week as a single phrase.

下个星期 = next week
下星期 is also widely used, especially in speech.

So:

下个星期 – slightly more complete/formal
下星期 – very common in everyday spoken Mandarin

Both are correct. You will hear both; you can safely use either.

What is the difference between 下个星期, 下星期, 下周, and 下礼拜?

All can mean next week, with style and regional preferences:

下个星期 – standard, neutral, very common in textbooks
下星期 – standard, common in everyday speech
下周 – a bit shorter, feels more written/official or used in news/business
下礼拜 – more colloquial; very common in Taiwan and some southern regions

For a learner, 下个星期 and 下星期 are the safest, most neutral choices.

There is no word meaning will in this sentence. How does Chinese show the future here?

Mandarin usually shows time through:

time words: 下个星期 (next week), 明天 (tomorrow), etc.
context
sometimes auxiliary verbs like 会, 要, 打算

In 下个星期我们决定一起去公园。, the future meaning comes from:

下个星期 – clearly future time
决定 – the decision is in place now, about a future action

You could add an explicit future marker, but it is not necessary:

下个星期我们会一起去公园。
下个星期我们要一起去公园。

The original sentence is fully natural and already clearly future.

Why do we say 我们决定一起去公园 and not something like 我们有一个决定去公园?

决定 in Mandarin is both:

a verb: to decide
a noun: a decision

In your sentence, it is a verb:

我们决定一起去公园。
Subject (我们) + Verb (决定) + Verb phrase (一起去公园)

Structure: 决定 + do something = decide to do something.

If you really want to use 决定 as a noun, you need a different structure:

我们做了一个去公园的决定。
(We made a decision to go to the park.)

But that is more formal and wordy. The simple 我们决定一起去公园 is the most natural way to say we’ve decided to go to the park together.

Should there be a 了 after 决定? What is the difference between 我们决定一起去公园 and 我们决定了一起去公园?

Both are possible, but the nuance is slightly different.

我们决定一起去公园。
Neutral statement: we decided / have decided to go to the park together. Very common.
我们决定了一起去公园。
Emphasizes that the act of deciding is completed, or contrasts with not having decided before. It can sound like:
- we have (finally) made a decision: to go to the park together
- or you are stressing the decision itself in a particular context

In everyday speech, for simple statements like this, learners can safely use 我们决定一起去公园。 without 了.

How does 一起 work here? Why is it before 去公园 and not somewhere else?

一起 means together and usually goes before the main action verb:

一起 + Verb

So:

一起去公园 = to go to the park together

Common positions for 一起:

After the subject:
- 我们一起去公园。
  We go to the park together.
After higher-level verbs that introduce an action, like 决定:
- 我们决定一起去公园。
  We decided to go to the park together.

我们一起决定去公园。 has a different meaning:

We together made the decision to go to the park.
(Emphasis on making the decision together, not on going together.)

我们决定去公园一起 is unnatural; 一起 almost always comes before the verb it modifies.

Why do we use 去 and not 到 in 去公园? What is the difference?

Both 去 and 到 can relate to going somewhere, but:

去 is a verb: to go (away from the speaker)
到 can be:
- a verb: to arrive
- a coverb/preposition: to, until

In everyday go somewhere sentences, the usual pattern is:

去 + place
去公园 – go to the park

到 is more about arrival or completion:

到公园去玩 – go to the park to play (literally: arrive at the park and then go play)
我们已经到公园了。 – we have arrived at the park.

In simple sentences meaning to go to a place, 去公园 is the normal, simple choice.

Why doesn’t 公园 need any extra word like 的 or a preposition before it?

In 去公园, 公园 is a direct object of the verb 去:

Verb (去) + Object (公园) = go (to) the park

Chinese does not need a separate preposition here, because 去 itself already implies movement to a place. So:

去北京 – go to Beijing
去学校 – go to school
去公园 – go to the park

No 的 is used between a main verb and its direct object. 的 usually links modifiers and nouns (e.g. 漂亮的公园 – beautiful park), not verbs and objects.

Could I say 我们下个星期决定一起去公园 instead? Is that still correct?

Yes, that is also correct:

我们下个星期决定一起去公园。

The difference is just word order and slight emphasis:

下个星期我们决定一起去公园。
First highlight the time: next week, we have decided to go...
我们下个星期决定一起去公园。
First mention we, then add the time phrase in the middle.

Both sound natural. In conversation, putting the time after the subject is extremely common. Both patterns are worth practicing.

How are the tones pronounced in this sentence, especially 一起 and 个?

Word-by-word dictionary tones:

下 xià – 4th tone
个 gè – 4th tone
星期 xīngqī – 1st + 1st
我们 wǒmen – 3rd + neutral
决定 juédìng – 2nd + 4th
一起 yìqǐ – 4th + 3rd (dictionary form)
去 qù – 4th
公园 gōngyuán – 1st + 2nd

Tone sandhi and natural pronunciation:

一起
- In real speech, 一 before a 3rd-tone syllable is pronounced with 2nd tone.
- So 一起 is pronounced yíqǐ (2nd + 3rd) in natural speech.
个
- Learners should learn it as 4th tone gè.
- In fast, casual speech it can sound lighter, but the basic tone is 4th.

So a natural reading would be:

xià ge xīngqī wǒmen juédìng yíqǐ qù gōngyuán.

Why do we use 我们 here? Could Chinese drop the subject like in some other sentences?

Mandarin can drop pronouns when they are obvious from context. But in an isolated sentence like this, saying 我们 makes it clear that:

It is we, not I, who decided.

Variations:

我决定下个星期一起去公园。
I decided to go to the park together next week. (Implies with someone, but does not say who.)
(context known, spoken) 下个星期决定一起去公园。
If everyone already knows who is deciding, you might omit 我们 in very casual speech, but as a standalone example this would usually sound incomplete.

For clear, neutral Mandarin, keep 我们 here.

Do we ever need 是 in a sentence like this, such as 下个星期我们是决定一起去公园?

No, you should not add 是 here.

是 is the verb to be and is used mainly:

between two nouns/pronouns:
- 他是老师。 – He is a teacher.
in 是…的 constructions for emphasis on when/how/where something happened.

In your sentence, the main verb is already 决定 (to decide). You cannot normally have two main verbs (是 and 决定) in that slot.

下个星期我们决定一起去公园。 – correct
下个星期我们是决定一起去公园。 – incorrect in normal usage

You might see 我们是决定去公园的, but that is a special 是…的 structure with a different nuance, not needed for basic expression here.