El pneumàtic és nou.

Because pneumàtic is a masculine singular noun in Catalan.

• el = the for masculine singular nouns
• la = the for feminine singular nouns
• els / les = plural forms

So:

• el pneumàtic = the tire

If it were plural, it would be els pneumàtics.

És means is. It is the 3rd person singular form of the verb ser (to be).

So:

• El pneumàtic és nou = The tire is new

The accent matters because és and es are different words:

• és = is
• es = a pronoun used in reflexive structures

So the accent helps distinguish them.

Catalan, like Spanish, often distinguishes between ser and estar.

Here, és nou means the tire is new in the basic sense: it is a new tire, not an old or used one.

• és nou = it is new
• està nou = it is in like-new condition / it looks almost new

So ser is the normal choice when you are describing what something is like in an essential or defining way.

Because adjectives in Catalan usually agree in gender and number with the noun they describe.

Since pneumàtic is masculine singular, the adjective must also be masculine singular:

• nou = masculine singular
• nova = feminine singular
• nous = masculine plural
• noves = feminine plural

Examples:

• El pneumàtic és nou.
• La roda és nova.
• Els pneumàtics són nous.

It can mean either one, depending on context.

In this sentence, nou means new because it comes after és and works as an adjective:

• El pneumàtic és nou = The tire is new

As a number, nou means nine:

• nou pneumàtics = nine tires

So the sentence structure tells you which meaning is intended.

In Catalan, the initial p in pn- is usually pronounced, unlike in English pneumatic, where the p is silent.

A learner-friendly guide is:

• pneu-MÀ-tic

The accent in pneumàtic shows that the stress falls on mà.

So one important difference from English is:

• English: pneumatic → silent p
• Catalan: pneumàtic → p is normally pronounced

The accent in pneumàtic tells you where the stress goes:

• pneu-MÀ-tic

Without getting too technical, the written accent helps you pronounce the word correctly and shows that the stressed syllable is mà.

Catalan uses written accents more regularly than English, so they are important to pay attention to.

Pneumàtic is correct and standard, but in everyday Catalan many speakers also use pneu.

So both can appear:

• El pneumàtic és nou.
• El pneu és nou.

Pneu is shorter and very common in everyday contexts, especially when talking about vehicles.

Yes, it can.

In your sentence, nou comes after the verb:

• El pneumàtic és nou = The tire is new

But inside a noun phrase, the position of nou can matter:

• un pneumàtic nou = a new tire
• un nou pneumàtic = often another/new additional tire

So when nou comes before the noun, it can sometimes mean new in the sense of another or additional, not just unused.