امروز در پارک بازی نمیکنیم، چون خسته هستیم.

Because Persian often drops subject pronouns when the verb already makes the subject clear.

In نمی‌کنیم / نمیکنیم, the ending -یم means we.
In هستیم, the ending also shows we are.

So ما is not necessary, though you could say ما امروز در پارک بازی نمی‌کنیم، چون خسته هستیم for extra emphasis.

This is part of the compound verb بازی کردن, which means to play.

So:

• بازی = play, game
• کردن = to do
• بازی کردن = to play

Then نمی‌کنیم breaks down like this:

• نمی- = present negative marker
• کن = present stem of کردن
• -یم = we

So literally it is something like we do not do play, but naturally it means we do not play.

Persian uses many compound verbs, where a noun, adjective, or other element combines with a light verb such as کردن, شدن, دادن, or گرفتن.

بازی کردن is just the normal way to say to play in many contexts.

This is very common in Persian, so learners should get used to seeing:

• a non-verbal element first
• the conjugated light verb at the end

Persian usually follows Subject–Object–Verb order, and the main verb normally comes at the end.

In this sentence, the structure is roughly:

• امروز = today
• در پارک = in the park
• بازی نمی‌کنیم = we do not play
• چون خسته هستیم = because we are tired

So the main idea comes before the reason, and both verbs appear toward the ends of their clauses.

Time expressions often come early in Persian, especially when they set the scene for the whole sentence.

So امروز at the beginning sounds very natural:

• امروز در پارک بازی نمی‌کنیم...

But Persian word order is somewhat flexible. You could also hear other arrangements, depending on emphasis.

For example:

• در پارک امروز بازی نمی‌کنیم...
• بازی امروز در پارک نمی‌کنیم...
  though this is less neutral and depends more on context

The version in your sentence is the most straightforward and natural.

در means in or inside. So در پارک means in the park.

This is correct and neutral. In everyday spoken Persian, people also often use:

• تو پارک
• توی پارک

These are more conversational ways to say in the park.

So:

• در پارک = a bit more formal or standard
• تو/توی پارک = more colloquial

چون means because and introduces the reason clause.

So:

• چون خسته هستیم = because we are tired

This clause explains why we are not playing in the park today.

You can also reverse the order of the clauses:

• چون خسته هستیم، امروز در پارک بازی نمی‌کنیم.

That still means the same thing, just with the reason first.

Because خسته is an adjective, and Persian needs a form of to be to say we are tired.

So:

• خسته = tired
• هستیم = we are

Together:

• خسته هستیم = we are tired

This is like saying:

• او خسته است = he/she is tired
• من خسته هستم = I am tired
• ما خسته هستیم = we are tired

Yes. Both are correct.

• خسته هستیم
• خسته‌ایم

Both mean we are tired.

The second version is more compact because the to be element is attached more closely to the adjective. In many cases, Persian allows both a fuller form and a shorter attached form.

Learners should recognize both as normal.

The negative applies only to the first verb:

• بازی نمی‌کنیم = we do not play
• خسته هستیم = we are tired

So the meaning is:

• We are not playing today
• The reason is that we are tired

It does not mean we are not tired.

If you wanted to say we are not tired, you would negate the second clause instead:

• خسته نیستیم

The standard spelling is نمی‌کنیم, with a separator between نمی and کنیم.

In informal typing, many people write نمیکنیم without that separator. So both may appear online or in casual text, but نمی‌کنیم is the standard form you should learn for careful writing.

The same applies to many other verbs with می- and نمی-.

Not always, but it is common and helpful.

Persian punctuation is somewhat flexible, especially in less formal writing. The comma here helps show that the second clause gives the reason:

• امروز در پارک بازی نمی‌کنیم، چون خسته هستیم.

Without the comma, the sentence is still understandable:

• امروز در پارک بازی نمی‌کنیم چون خسته هستیم.

So the comma is useful, but not absolutely required in every context.