Learn German: Es jubeln viele Leute über die Vorstellung. - Many people are cheering about the performance.

Here, Es is a placeholder or dummy subject. It takes up the first position in the sentence so that the real subject, viele Leute, can be delayed until after the verb. This structure is similar to saying 'There are many people cheering...' in English, and it is often used to introduce a scene or put focus on the action itself.

If the sentence starts with Es, why is the verb jubeln in the plural form instead of the singular jubelt?

The verb must always agree with the true logical subject of the sentence, not the placeholder Es. Since the real subject is the plural viele Leute, the verb takes the plural form jubeln.

Can I reword this sentence to put viele Leute at the beginning, and if so, where does the es go?

Yes, you can start with Viele Leute. If you do, the placeholder es completely disappears. The sentence simply becomes Viele Leute jubeln über die Vorstellung. The dummy es only exists to fill Position 1 when nothing else is there, and it vanishes if another word takes that spot.

Why does über take the accusative die Vorstellung in this sentence?

The phrase for cheering about something is jubeln über. While über is a two-way preposition that can take the dative case for physical locations, it almost always takes the accusative case when used abstractly to mean 'about'.