Learn Farsi: پروفسور موسوی گردهمایی سالانه را شروع کرد. - Professor Mousavi started the annual convention.

While native Farsi titles like آقا (Mr.) and خانم (Ms./Mrs.) take the explanatory Ezafe before a name, academic and professional titles borrowed from Western languages typically do not. Therefore, we say پروفسور موسوی (Porofosor Mousavi) and دکتر موسوی (Doktor Mousavi) without connecting them with an Ezafe.

How does the word گردهمایی translate to convention?

This compound noun is built from گرد (around) and هم (together), combined with آیی (a noun-forming suffix from the verb root for coming). Literally meaning a coming together around something, it is the standard term for a gathering, summit, or convention.

Why does the object marker را come after سالانه instead of right after گردهمایی?

The object marker را must always follow the complete noun phrase. Because سالانه (annual) is an adjective linked to گردهمایی (convention) by an Ezafe, they form a single descriptive block. The marker attaches to the very end of this entire direct object block.

Would it be natural to use آغاز کرد instead of شروع کرد in this sentence?

Yes. آغاز کردن means the exact same thing but feels slightly more formal and literary. Given that a professor is opening an official annual convention, using the more formal verb would sound very natural and appropriate here.