Learn Farsi: اگر مواظب نباشید، فریب آن کلاهبردار را می‌خورید. - If you are not careful, you will be deceived by that scammer.

In Farsi, some compound verbs form their passive not by using shodan (to become), but by swapping their light verb. The active verb for 'to deceive' is farib dâdan (literally 'to give deception'). To make it passive ('to be deceived'), we replace dâdan with khordan (to eat/suffer), creating farib khordan.

If the verb is translated as passive ('to be deceived'), why is the direct object marker râ used after the scammer?

Even though the English translation is passive, the Farsi structure is technically active. You are literally saying 'you will eat/suffer the deception of that scammer'. Because 'the deception of that scammer' (farib-e ân kolâhbardâr) is a specific, defined thing that you are 'eating', it requires the specific direct object marker râ.

How is the word 'by' expressed in 'deceived by that scammer'?

It isn't expressed with a preposition like az (from/by) in this specific structure. Instead, Farsi uses the ezafe (the invisible 'e' linking words). farib-e ân kolâhbardâr means 'the deception of that scammer'. By 'suffering the deception OF the scammer', you are effectively being deceived BY them.

Why is the first verb subjunctive (nabâshid) but the second verb present indicative (mikhorid)?

This is a standard 'Type 1' conditional sentence, which talks about a realistic possibility. The 'if' clause with agar requires the subjunctive () because the condition hasn't happened yet and is hypothetical. The result clause uses the present indicative () to confidently state what will happen, which is very common for near-future results in Farsi.