من در را قفل نکردم، چون منتظر خواهرم بودم.

را marks a specific direct object. Here, در را means the door as the thing being locked.

So:

• در = door
• در را = the door / the door as the object of the verb

In this sentence, the speaker means a particular door, not just a door in general.

In everyday spoken Persian, را is often pronounced ro or o, so you may hear:

• در رو قفل نکردم

instead of the more formal written form:

• در را قفل نکردم

Yes, من can be omitted.

Persian verbs usually show the subject clearly, so کردم and بودم already tell you the subject is I. That means the sentence could also be:

• در را قفل نکردم، چون منتظر خواهرم بودم.

Including من adds a little emphasis or clarity, similar to saying I didn’t lock the door... rather than just didn’t lock the door...

قفل کردن is a compound verb meaning to lock.

It is made of:

• قفل = lock
• کردن = to do

Together: قفل کردن = to lock

Then نکردم is the negative past of کردن for I:

• کردم = I did
• نکردم = I did not do

So:

• قفل کردم = I locked
• قفل نکردم = I did not lock

This is very common in Persian: many actions are expressed with a noun/adjective + کردن.

It can mean both, but the context makes it clear.

• در = door as a noun
• در = in / at / inside as a preposition

In this sentence, در را قفل نکردم clearly uses در as door, because:

• it is followed by را, which marks a noun object
• it goes naturally with قفل کردن = to lock

So here در cannot mean in.

Persian is normally Subject-Object-Verb in basic word order.

So:

• من = subject
• در را = object
• قفل نکردم = verb

Literally, the first clause is closer to:

• I the door did not lock

This is completely normal in Persian. The same happens in the second clause:

• منتظر خواهرم بودم
• literally: waiting for my sister was-I

چون means because here.

So the sentence structure is:

• I didn’t lock the door, because I was waiting for my sister.

A few notes:

• چون is very common in everyday Persian.
• Other words can also mean because, such as زیرا or به خاطر اینکه, but چون is simple and natural here.

In Persian, منتظر is an adjective meaning waiting / expectant, and it is often used with بودن:

• منتظر بودن = to be waiting

So:

• منتظر = waiting
• بودم = I was

Together:

• منتظر خواهرم بودم = I was waiting for my sister

This is slightly different from using a simple verb like English waited, but it is a very normal Persian pattern.

The -م at the end is a possessive ending meaning my.

So:

• خواهر = sister
• خواهرم = my sister

Other common possessive endings are:

• خواهرت = your sister
• خواهرش = his/her sister
• خواهرمان = our sister
• خواهرتان = your sister (plural/formal)
• خواهرشان = their sister

Because منتظر can take its complement directly.

In English, we say:

• waiting for my sister

In Persian, you can simply say:

• منتظر خواهرم

So Persian does not need a separate word equivalent to for here.

That is a common difference between English and Persian: some words that need a preposition in English do not need one in Persian.

بودم is the past tense of to be for I:

• بودم = I was

So:

• منتظر خواهرم بودم = I was waiting for my sister

This gives background information in the past, explaining why the speaker did not lock the door.

Not always.

In Persian writing, punctuation is somewhat flexible, and many people do write a comma before چون in a sentence like this:

• من در را قفل نکردم، چون منتظر خواهرم بودم.

But you may also see it without a comma:

• من در را قفل نکردم چون منتظر خواهرم بودم.

Both are understandable. The comma just helps show the pause and the reason clause more clearly.

Yes. In spoken Persian, a very common version would be:

• من در رو قفل نکردم، چون منتظر خواهرم بودم.

The main difference is:

• را in formal writing
• رو in speech

You might also drop من in casual speech:

• در رو قفل نکردم، چون منتظر خواهرم بودم.

That sounds very natural.

It mainly gives a reason:

• I didn’t lock the door because I was waiting for my sister.

Depending on tone and context, it can also sound like an explanation or even a mild excuse. Grammatically, though, it simply states the reason for not locking the door.

So the sentence does not necessarily mean the speaker forgot; it suggests the door was intentionally left unlocked because the sister was expected.