من آن پیراهن را پوشیدم، اما شلوار را نخریدم چون قیمتش زیاد بود.

Yes—Persian is often pro-drop, so من is not strictly necessary here because پوشیدم and نخریدم already show I through the ending -م.

So all of these are possible:

• من آن پیراهن را پوشیدم
• آن پیراهن را پوشیدم

Including من can add a little emphasis, clarity, or contrast. In this sentence, it is perfectly natural, but it could also be omitted.

را is the direct object marker. It usually marks a specific or definite direct object.

So:

• آن پیراهن را = that shirt as the object
• شلوار را = the pants / the trousers as the object

It does not have a direct English translation. It is more of a grammar marker than a vocabulary word.

A useful contrast:

• پیراهن خریدم = I bought a shirt
• پیراهن را خریدم = I bought the shirt

Persian does not have a true definite article like English the.

Definiteness is usually understood from context, or shown by things like:

• آن = that
• این = this
• را after a specific object
• possessives, context, previous mention, etc.

So شلوار را can naturally mean the pants/trousers even though there is no separate word meaning the.

In Persian, demonstratives like این and آن normally come before the noun:

• این کتاب = this book
• آن پیراهن = that shirt

So آن پیراهن is the normal word order.

پیراهن آن would mean something different, more like his/her shirt only if آن were being used in a special literary sense, and in normal modern Persian it would not be the way to say that shirt.

پوشیدم comes from the verb پوشیدن.

In the simple past:

• past stem: پوشید
• -م = I

So:

• پوشیدم = I wore / I put on

Depending on context, پوشیدن can mean either to wear or to put on. Persian often uses the same verb where English may choose different ones.

نخریدم comes from خریدن = to buy.

It breaks down like this:

• خرید = past stem
• نـ = negative prefix
• -م = I

So:

• خریدم = I bought
• نخریدم = I did not buy

This is a very common pattern in Persian past tense verbs.

Because Persian treats شلوار as a singular noun meaning one pair of pants/trousers.

So:

• شلوار = pants / trousers / a pair of pants
• شلوارها = pants/trousers in the plural, meaning multiple pairs

This is just a difference between the two languages. English uses a plural form for one garment; Persian does not.

قیمتش means its price or the price of it.

It is made of:

• قیمت = price
• -ش = his / her / its

So:

• قیمتش زیاد بود = its price was high

This -ش is an attached pronoun, very common in Persian. Similar examples:

• کتابش = his/her book
• اسمش = his/her/its name
• دوستش = his/her friend

Both are possible, but they are slightly different in focus.

• شلوار گران بود = the pants were expensive
• قیمتش زیاد بود = its price was high

The sentence you have focuses on the price, not directly on the item itself.

Also, زیاد بودن قیمت is a very natural Persian way to express that something costs too much. English often prefers expensive, but Persian often talks about the price being high.

چون means because.

In this sentence:

• شلوار را نخریدم چون قیمتش زیاد بود
• = I didn’t buy the pants because its price was high

Yes, the چون clause can move:

• چون قیمتش زیاد بود، شلوار را نخریدم.

That is also natural and means the same thing. The difference is mostly about emphasis and flow.

آن is correct and natural, but in everyday spoken Persian people very often say اون instead.

So in conversation, you are likely to hear:

• اون پیراهن رو پوشیدم، اما شلوار رو نخریدم چون قیمتش زیاد بود.

This is the colloquial version of the same sentence.

A few spoken changes there are:

• آن → اون
• را → رو
• sometimes pronunciation becomes more relaxed overall

So your sentence is standard written Persian, and the spoken version is slightly different but very common.