Learn Polish: Ci dorośli ludzie żyli w Warszawie. - These adult people lived in Warsaw.

The singular form is człowiek (person / human). This is one of the most irregular plurals in Polish! Instead of just adding a normal plural ending, the entire word changes to ludzie (people).

Why does this sentence use ci instead of te for "these"?

In Polish plurals, we divide nouns into "masculine personal" (groups containing at least one male person) and "non-masculine personal" (everything else). Because ludzie means "people" and implies a mixed or male group, it takes the masculine personal word for "these": ci. If we were pointing at a group of only women, animals, or objects, we would use te.

I learned the word for "adult" is dorosły. Why does it change to dorośli here?

Because ludzie is a masculine personal plural noun, the adjective describing it must also take the masculine personal plural form. For adjectives ending in -sły (like dorosły), the ending changes to -śli in this form. This softening makes the word easier to say with the plural -i ending.

Why does the verb end in -li (żyli) instead of -ły (żyły)?