Learn Portuguese (Portugal): As calças cinzentas são caras.

Just like "trousers" or "pants" in English, calças is a plural word when talking about a single pair. In Portuguese, it is also a feminine word. Therefore, to say "the", you must use the feminine plural article As.

Why does the word for "grey" end in "-as" (cinzentas)?

Adjectives in Portuguese have to match the noun they describe in gender (masculine/feminine) and number (singular/plural). Because calças is feminine and plural, the default dictionary word for grey (cinzento) changes to cinzentas. Notice also that the adjective comes directly after the noun, which is the standard word order in Portuguese.

Why use são (from the verb ser) instead of estão (from the verb estar)? Don't prices change?

While it's true a store might have a sale, being "expensive" or "cheap" is treated grammatically as an inherent, defining quality of the item itself. Because it's describing a built-in characteristic rather than a temporary state, we use the verb ser, which gives us the "they are" form: são.

Does the word for expensive (caras) change for the same reason?

Exactly! Just like the color grey in this sentence, the adjective for "expensive" must agree with As calças. The base word is caro, and we change the ending to make it feminine and plural, resulting in caras.