Learn Farsi: خواهر من نمی‌تواند این نامه را ترجمه کند.

Because of the modal verb نمی‌تواند (cannot). In Farsi, when you use a verb like 'can' or 'cannot', the action that follows it must be in the present subjunctive form. For compound verbs made with کردن (to do/make), we just drop the می- prefix. So می‌کند becomes کند.

How is the word for 'cannot' built in this sentence?

The word نمی‌تواند is built from the verb توانستن (to be able to). Since the subject is 'my sister' (she), it takes the 'she/he' ending ـد. The negative prefix نـ (ne-) is added right before the present tense prefix می (mi-), making it نمی‌تواند (nemi-tavânad).

Why do we need the word را after 'this letter'?

The word را (râ) is an important marker that points out a specific direct object. Because we are talking about this specific letter (این نامه) and not just any random letter, we must use را right after it to show exactly what is receiving the action of being translated.

I see the words for 'sister' and 'I', but how do they mean 'my sister'?

They are connected by the Ezafe, which is a short 'e' sound that links nouns to their owners or adjectives. You pronounce it as khâhar-e man. Even though the '-e' sound connects خواهر (sister) and من (I/me) to mean 'my sister', it is usually not written out in the Farsi alphabet.