Learn Farsi: کدام پیراهن را دوست داری، قرمز یا آبی؟ - Which shirt do you like, the red one or the blue one?

Yes! In Farsi, when you use کدام (which) to ask about a specific item from a group, the noun that follows it must be singular. So it is always کدام پیراهن (which shirt) and never plural, even though you are choosing from multiple shirts.

Why do we need the word را after پیراهن in this sentence?

The word را marks the specific direct object of a sentence. Because you are asking about a specific shirt using the question word کدام (which), the noun phrase کدام پیراهن becomes specific. Therefore, it needs the object marker را.

The English translation says 'the red one or the blue one', but I cannot find a word for 'one' in the Farsi sentence. Where is it?

In Farsi, you do not need a placeholder word like 'one'. You simply use the adjectives قرمز (red) and آبی (blue) on their own. They act exactly like nouns here, so saying قرمز یا آبی literally means 'red or blue' but naturally translates to 'the red one or the blue one'.

Why does the verb دوست داری appear in the middle of the sentence instead of at the very end?

Farsi typically puts the verb at the end of the clause. Here, دوست داری (you like) is at the end of the main clause. The color choices قرمز یا آبی (red or blue) are just added on as a clarification. This conversational add-on structure is very natural and common in Farsi.