A área comum é grande.

Because a is the definite article: the.

So a área comum means the common area, not a common area.

• a área comum = the common area
• uma área comum = a common area

Portuguese uses definite articles very often, especially when referring to a specific thing already known from context.

Área is a feminine noun, so it takes the feminine singular article a.

• a área
• not o área

Many Portuguese nouns ending in -a are feminine, and área follows that pattern.

A useful thing to remember:

• o = masculine singular
• a = feminine singular
• os = masculine plural
• as = feminine plural

In Portuguese, adjectives often come after the noun.

So:

• área comum = common area

This is more natural than putting the adjective first.

English usually says adjective + noun:

• common area

Portuguese very often says noun + adjective:

• área comum

That said, adjective position in Portuguese can vary, but in this case área comum is the standard order.

Because comum is an adjective that has the same form for masculine and feminine singular.

So you can say:

• o espaço comum
• a área comum

In the singular, the form stays comum.

It only changes in the plural:

• áreas comuns
• espaços comuns

So the pattern is:

• singular: comum
• plural: comuns

É is the third-person singular form of the verb ser in the present tense.

Here, the subject is a área comum, which is singular, so the verb is singular too:

• A área comum é grande.

Compare:

• A área comum é grande. = singular
• As áreas comuns são grandes. = plural

So é means is here.

The accent in é is important because it distinguishes this word from e.

• é = is
• e = and

So:

• A área comum é grande. = The common area is big.
• If you wrote e instead, it would be a different word entirely.

The accent shows both meaning and pronunciation.

The accent in área helps show the correct stress and pronunciation.

It is pronounced roughly as AH-re-a, with the stress on the first syllable.

Without getting too technical, the accent tells you that the word should not be read with the stress somewhere else. It helps mark the correct spoken rhythm of the word.

So:

• área → stress on Á

Yes, grande agrees in number, but not in gender in the singular.

That means:

• a área grande
• o prédio grande

In both cases, it stays grande.

But in the plural:

• áreas grandes
• prédios grandes

So grande does not change for masculine vs. feminine in the singular, but it does change for singular vs. plural.

Usually, no—not in normal standard Portuguese for this sentence.

The most natural form is:

• A área comum é grande.

Leaving out the article can sound incomplete or unnatural here.

Portuguese uses articles more often than English does, especially before nouns that refer to specific things.

A natural Brazilian Portuguese pronunciation is approximately:

A área comum é grande
ah AH-re-a ko-MUM EH GRAN-jee

A few notes:

• área has stress on the first syllable: Á
• comum has stress on the last syllable
• é is a clear eh
• grande in Brazilian Portuguese often sounds roughly like GRAN-jee

This is only an approximation for English speakers, but it can help you get started.

It becomes:

As áreas comuns são grandes.

Here is what changes:

• a → as
• área → áreas
• comum → comuns
• é → são
• grande → grandes

So Portuguese shows plural in several places, not just one.

Yes, but that means something slightly different in emphasis.

• A área comum é grande.
  This treats common area as a unit: the area being talked about is the shared/common area.

• A área é comum e grande.
  This says the area is common and big, as if common and big are two separate descriptions.

So if you mean the common area is big, the original sentence is the most natural choice.