Learn Italian: Quella montagna è grande.

In Italian, the word for "that" changes its ending to match the noun it describes, similar to how words for "the" work. Because montagna is a feminine singular noun starting with a consonant (which would take the article la), the word for "that" becomes quella.

Since montagna is feminine, why doesn't the adjective grande end in -a?

While many Italian adjectives change to end in -a for feminine words, grande belongs to a different group. Adjectives that end in -e in the dictionary are used for both masculine and feminine singular nouns. It stays grande whether you are talking about a feminine mountain or a masculine book.

If montagna started with a vowel, would we still use quella?

No. If you wanted to say "that" with a feminine noun starting with a vowel, like isola (island), you would drop the final -a and add an apostrophe, making it quell'isola. Since montagna starts with the consonant m, quella keeps its full form.

Why does the word è have an accent mark?