Learn Hardcore Korean: sueobeul sijakhaeyo.

Korean frequently drops the subject when it’s clear from context. In a classroom scenario, who’s starting the class (teacher or host) is obvious, so you don’t need to say 저는 or 선생님이.

What function does the particle 을 have after 수업?

을 is the direct-object marker. It tells you that 수업 (class/lesson) is what’s being acted upon by the verb 시작해요.

How do you get 시작해요 from the dictionary form 시작하다?

Drop the final -다 from 시작하다 → 시작하-
Since 하다-verbs form the polite present with -해요, change 하 to 해요 → 시작해요

What’s the role of the polite ending -요 in 시작해요?

Adding -요 makes the sentence polite and suitable for everyday situations with people you’re not extremely close to (e.g., students to a teacher, strangers, colleagues).

What’s the difference between 수업을 시작해요 and 수업이 시작해요?