Learn Spanish (Latin America): Las tiendas de la ciudad son grandes.

Tiendas (stores) is a feminine noun. In Spanish, plural definite articles (words for "the") have to match both the gender and the number of the noun. Since tiendas is plural and feminine, we use las. If we were talking about a masculine plural noun, like mercados (markets), we would use los.

The English translation says "city stores", but the Spanish says tiendas de la ciudad. Why is it phrased this way?

In English, we often put a noun right before another noun to describe it (like "city" before "stores"). Spanish doesn't do this. Instead, you have to use the word de (of). So, you literally say "the stores of the city", which translates to las tiendas de la ciudad.

Why is the word for big grandes? Shouldn't it end in -as to match the feminine word tiendas?

Not all adjectives change their ending to -o or -a. Adjectives that end in -e in their singular form, like grande (big), do not have separate masculine and feminine forms. To make them plural, you just add an -s. So, grande becomes grandes to match the plural tiendas.

Why do we use son here instead of están?