Learn Portuguese (Portugal): Os carros são novos.

Since carro is a masculine noun ending in a vowel, we make it plural by simply adding an "s" to get carros. To say "the cars", we must also change the singular masculine article O to its plural form, Os.

How do we know to use são instead of another form of "to be"?

São is the "they" form of the verb ser (to be). Because os carros (the cars) is a plural subject, it acts like "they" (eles). Therefore, we use são just like we would say eles são (they are).

Why does the word for "new" change to novos?

In Portuguese, describing words (adjectives) must match the noun they describe. Because carros is masculine and plural, the adjective novo (new) also has to become masculine and plural. We just add an "s" to the end to make it novos.

Why do we use a form of ser (são) here instead of estar (estão)?