Learn Italian: L'auto è nuova.

In Italian, when the singular feminine article la comes right before a word starting with a vowel, the 'a' is dropped and replaced with an apostrophe. This makes the words flow together smoothly. So, la auto becomes l'auto.

Why does auto end in 'o' but use the feminine adjective nuova?

Auto is short for automobile, which is a feminine word. Because it is an abbreviation, it keeps its feminine gender even though it ends in 'o' instead of a typical feminine ending. That is why the adjective must be the feminine nuova instead of the masculine nuovo.

How would I know if l'auto is singular or plural here?

The word auto is an abbreviation, which means it doesn't change its ending in the plural. You have to look at the rest of the sentence! Here, the verb è (is) and the adjective nuova (new) are both singular, telling us there is only one car. If it were plural, the sentence would be le auto sono nuove.