Learn Farsi: او فقط خودش را گول می‌زند.

خود (khod) means 'self', and -ش (-ash) is the possessive ending for 'his/her/its'. Put them together, and you get 'herself' (or 'himself'). Because she is the direct object (the person being deceived), we must add the specific object marker را (râ) right after it.

The verb گول می‌زند translates to 'deceives', but doesn't می‌زند mean 'hits'?

Yes! This is a compound verb. گول (gul) means 'deceit' or 'trick', and the verb زدن (zadan) means 'to hit'. In Farsi, 'to hit a trick' (گول زدن) is the normal way to say 'to deceive' or 'to fool'. Here, it is conjugated for 'she' in the present tense.

How do we know the sentence means 'She ... herself' instead of 'He ... himself'?

By looking at the Farsi alone, you don't! Both the pronoun او (u) and the ending -ش (-ash) are gender-neutral. This sentence could just as easily mean 'He only deceives himself.' We just used 'She' in the English translation as one valid option.

Could we say او فقط خود را گول می‌زند without the -ش on the end of خود?

Yes, in more formal or literary Farsi, you can just use خود را (khod râ) for any person to mean the action is done to oneself. However, in everyday spoken and written Farsi, it is much more natural to add the matching personal ending to . Since the subject is 'she', we use to make it .