Learn Italian: Chi ha lasciato la spazzatura nello spazio comune? - Who left the garbage in the shared space?

Nello is the combination of the preposition in plus the definite article lo. We must use lo (and therefore nello) because the following noun, spazio, begins with an s followed by a consonant.

When using chi (who), do we always use the he/she verb form?

Yes! Just like in English, when chi is the subject of the sentence, it takes the third-person singular verb form. So we say chi ha lasciato (who left), even if we suspect multiple people did it.

Since this lesson is about independent possessive pronouns, how would someone reply to this by saying: They left it in ours?

They would use the pronoun il nostro (ours). Because spazio is masculine singular, you use the masculine singular pronoun. When you combine the preposition in with il nostro, it becomes nel nostro.

Spazio is masculine, so why does the adjective comune end in -e?

Comune belongs to a group of Italian adjectives that naturally end in -e in the singular. These adjectives use the exact same ending for both masculine and feminine nouns. If the noun were plural, the ending would change to -i (for example, gli spazi comuni).