A despesa variável não é uma despesa imprevista.

Because they are two different articles:

• a despesa = the expense
• uma despesa = an expense / one expense

In Portuguese, the definite article a is often used for general statements about a whole category. So A despesa variável... can talk about the category of variable expenses, not just one specific expense.

The second part, uma despesa imprevista, presents it as a type or instance: an unexpected expense.

You can tell from the words around it:

• a despesa
• uma despesa
• despesa imprevista

Those are all feminine forms. In Portuguese, nouns have grammatical gender, and despesa is a feminine noun. That is why it takes feminine articles and feminine adjective agreement when needed.

There is not always a logical reason a noun is masculine or feminine, so this is something you usually learn with the noun.

Because in Portuguese, adjectives very often come after the noun.

So:

• despesa variável
• despesa imprevista

is the most neutral, standard order.

Portuguese can sometimes place adjectives before the noun, but that often changes the tone or emphasis. In this sentence, the post-noun position is the normal choice.

Not all adjectives show gender the same way.

• imprevisto → masculine
• imprevista → feminine

But variável has the same form for masculine and feminine in the singular:

• custo variável
• despesa variável

It only changes in the plural:

• custos variáveis
• despesas variáveis

So variável does agree with the noun, but its masculine and feminine singular forms happen to look identical.

Because this sentence is defining or classifying something, and Portuguese normally uses ser for that.

• é = form of ser
• está = form of estar

Here, the sentence is saying that one category is not the same as another category. That is a job for ser:

• A despesa variável não é uma despesa imprevista.

Using estar would sound wrong in this context, because estar is usually for temporary states, conditions, or location.

That is the normal place for negation in Portuguese.

• não é = is not

In simple sentences, não usually goes directly before the conjugated verb:

• não é
• não tenho
• não podemos

So A despesa variável não é... follows the standard pattern.

It can be generic, meaning it refers to the category as a whole.

That is something English speakers often notice: Portuguese frequently uses the definite article where English would use no article or sometimes a/an in a general statement.

So A despesa variável can be understood as a general concept, not necessarily one specific expense.

Yes, you could say A despesa variável não é imprevista, but it is not exactly the same structure.

Compare:

• A despesa variável não é uma despesa imprevista.
• A despesa variável não é imprevista.

The original sentence compares two noun categories:

• variable expense
• unexpected expense

The shorter version simply says that the variable expense is not unexpected.

So both are grammatical, but the original is more explicit and more clearly contrasts two types of expenses.

You might see something like that in very condensed writing, titles, headings, or note-style language, but in normal full sentences, Brazilian Portuguese usually prefers the articles:

• A despesa variável não é uma despesa imprevista.

Leaving out both articles makes the sentence sound more abstract, technical, or slogan-like. For ordinary neutral speech, keeping the articles is the safest choice.

They are important.

• não: the tilde marks nasal pronunciation
• é: the accent marks stress and distinguishes it from e meaning and
• variável: the accent shows where the stress goes

So these marks are not optional decoration. They help show pronunciation and, sometimes, meaning.

For example:

• é = is
• e = and

That is a very important difference.

A rough English-friendly approximation is:

ah des-PEH-zah vah-ree-AH-vel now̃ EH oo-mah des-PEH-zah eem-preh-VEES-tah

A few key points:

• despesa: stress on pe
• variável: stress on á
• não: nasal sound; there is no strong final n
• é: open eh
• imprevista: stress on vis

Also, in many Brazilian accents, the final l in variável sounds a bit like an English w.

The basic structure is:

[noun phrase] + não é + [noun phrase]

More specifically:

• A despesa variável = subject
• não é = negative form of is
• uma despesa imprevista = predicate noun phrase

So the sentence is built as a negative identification/classification sentence: one thing is being said not to belong to another category.