Learn Italian: Molti cittadini partecipano alla protesta stasera. - Many citizens are participating in the protest tonight.

In Italian, the verb partecipare (to participate or take part) is followed by the preposition a. Because protesta is a feminine noun (la protesta), the preposition a combines with the definite article la to form alla.

Why use the present tense partecipano for an event happening tonight (stasera)?

Just like in English, where we can say 'Many citizens are participating tonight', Italian frequently uses the present tense to talk about the near future or a planned, certain event. You could use the future tense (parteciperanno), but the present is very natural here.

Does molti change its ending depending on the noun?

Yes. Here, molti (many) is an adjective modifying cittadini (citizens), which is a masculine plural noun. If it were describing a feminine plural noun, like persone (people), it would change to match it: molte persone.

Where does the word stasera come from?

Stasera is a common contraction of questa sera (this evening/tonight). You will also often see stamattina for questa mattina (this morning). Both forms are perfectly correct and widely used in everyday Italian.