Learn Italian: Il pesce è uno spuntino ottimo. - The fish is an excellent snack.

Because the next word, spuntino, starts with an "s" followed by a consonant (the letter "p"). In Italian, masculine words starting with this "s + consonant" combination (as well as letters like z, gn, and ps) take uno instead of un. It makes the words flow together more smoothly when you speak!

Since pesce ends in "-e", how can I tell if it is masculine or feminine?

Nouns ending in -e can be either masculine or feminine, so you often just have to memorize their gender. But this sentence gives you a huge clue: the word for "the" is the masculine article il. This tells you right away that pesce is a masculine noun.

Why does ottimo come after spuntino?

In Italian, adjectives (words that describe things) usually go after the noun they describe. So, instead of saying "an excellent snack" like in English, Italian phrases it as "a snack excellent" (uno spuntino ottimo).

Would ottimo change if it was describing a different word?

Yes! Adjectives must match the gender and number of the noun they describe. Because spuntino is a masculine singular noun, ottimo ends in -o. If you were describing a feminine singular noun, like a pizza, it would change to end in -a, becoming ottima.