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An object is almost always accompanied by an “object marker”. This marker is the word “را” which always comes right after the objects word(s) in the sentence.
کتاب را خواندم.
ما را دیدند.
علی کلید خانه‌اش را به من داد.
سارا و زهرا تکلیفشان را نوشتند.
I read the book.
They saw us.
Ali gave me the key of his house.
Sara and Zahra did (lit. wrote) their homework.
There are two places where there is an object in the sentence without an object marker. Understanding and mastering these two situations is one of the most complicated parts of Persian grammar.
1- Generic Nouns
When a noun refers to every member of a particular kind, group, species, etc, it is called a generic noun in that sentence. Generic nouns do not need object markers when they are used as objects. The word “sandwich” in the second sentence of the following examples is a generic noun. But it is a normal definite noun in the first sentence.
فقط ساندویچ را می‌خورد.
فقط ساندویچ می‌خورد.
He only eats the sandwich (and not the salad).
He only eats sandwiches.
However, when an adverbial clause comes between the generic noun and the verb of the sentence, the object marker should be used.
من همیشه ساندویچ را با دست می‌خورم.
I always eat sandwiches using (my) hands
(and not using a spoon).
2- Indefinite nouns
The object marker can be omitted when the object is indefinite.
لاله موهای بلندی دارد.
وبلاگ خوبی داری.
عکس‌های قشنگی می‌گیرد.
عکس‌های قشنگی را که گرفت به من داد.
Lale has long hairs.
You have a nice blog.
She takes nice photos.
She gave the nice photos she took to me.
In the last example, the object marker “را” is used. The reason is that as we explained in the previous lesson, the noun phrase is no longer considered indefinite when the word “که” is used after it to explain about it. It is definite and must therefore be accompanied by an object marker when taking the role of an object.

This is what one should know about definite and indefinite nouns and their usage:
Definite nouns: usually translated with “the”; without the suffix “ی” (unless followed by the word “که”); take the object marker when coming as an object
Indefinite nouns: usually translated with “a”, “an”, “some”, numbers or simply nothing; with the suffix “ی” or preceded by a number; usually without the object marker when coming as an object

Here are examples of different sentences with definite and indefinite objects:
بستنی‌ها را خوردند.
او پول من را دزدید.
من تهران را می‌شناسم.
ما همه‌چیز را می‌دانیم.
رئیس جمهور چیزهای عجیبی می‌گوید.
خودکار داری؟
همیشه تنها شام می‌خورد.
کتابی را که خریدی خواندم.
آنها کار سختی دارند.
They ate the ice-creams.
He stole my money.
I am familiar with Tehran.
We know everything.
The president says strange things.
Do you have a pen?
She always eats dinner alone.
I read the book you bought.
They have a hard job.
In the following examples, you can compare the two possible ways for using pronoun objects (Enclitic and free).
تو را می‌شناسم.
آن را گرفت.
ما را می‌بینند.
I know you.
I know you.
She took it.
She took it.
They see us.
They see us.
As mentioned in the previous lessons, the third person singular free personal pronoun is “او” for humans and “آن” for non-humans. When enclitic pronouns are being used, the third person pronoun is “ش” anyway, for both humans and non-humans. Thus, another case where we might prefer to use free pronouns is when we want to distinguish between human and non-human objects.
Word Order
It is usually said that the order of words in Persian sentences usually follow the SOV order (Subject-Object-Verb) but that is not obligatory. There are quite a lot of cases where the SOV rule is not observed; an at hand example is the group of sentences with enclitic objective pronouns which were just mentioned in the last set of examples.