Learn Italian: Questa macchina non è nuova.

In Italian, the negative word non must go directly before the verb it negates. Since the verb here is è (is), you have to say non è (literally "not is"). You cannot put non after the verb.

Why do both questa and nuova end in the letter "a"?

They both end in "a" to match the word they are describing. The word for car (macchina) is a feminine noun. In Italian, describing words must agree with the noun's gender, so the default masculine words questo (this) and nuovo (new) change to their feminine forms: questa and nuova.

I thought macchina literally translated to "machine". Why is it used for "car"?

While macchina does mean "machine", it is the most common, everyday word Italians use for "car". You can also use the word automobile (or just auto), but macchina is what you will hear most often in casual conversation.

If I wanted to talk about multiple cars, would the word non change?