I asked my teacher why is "が”はじまる instead of ”は”はじまる. She said "が” is always put before はじまる. But she said there are circumstances that ”は” is put before はじまる too. She said there are rules meanwhile couldn't clarify. Anyone can explain this? Many thanks!
が is used to report news or information recently discovered ("neutral description ga"). So ～が始まる is used when you want to tell a certain event will start now.
は is used to describe a general fact. ～は始まる is used to describe a fact that's related to the beginning of something. The described "fact" has nothing to do with what's happening in front of you.
- セサミストリートが始まります。 Sesami Street starts (now).
- セサミストリートは日曜日の朝9時に始まります。 Sesami Street starts at 9:00 Sunday morning.
- × セサミストリートは始まります。 (Makes no sense; sounds like "× Sesami Street is a thing that starts.")