A renda extra cobre a despesa fixa.

Because renda and despesa are both feminine singular nouns, so they take the feminine singular definite article a.

• a renda = the income
• a despesa = the expense

In Portuguese, articles are used more often than in English, so a sentence like this sounds natural with both articles included.

Noun gender in Portuguese is a grammatical feature, not always something logical. You simply learn each noun with its gender:

• a renda
• a despesa

A useful clue is that many nouns ending in -a are feminine, though this is not a perfect rule.

Because they are feminine, any adjectives that agree with them will usually also be feminine:

• despesa fixa
• not despesa fixo

Renda usually means income or earnings, especially in a personal, family, or household sense.

So renda extra means something like:

• extra income
• additional earnings
• side income

It is not usually the best word for every kind of business revenue. For business contexts, receita can also be used, depending on the meaning.

In Portuguese, adjectives often come after the noun, much more often than in English.

So:

• renda extra = extra income
• despesa fixa = fixed expense

That word order is the normal one here. Putting extra before renda would sound unnatural in this sentence.

Not in the singular here. Extra can describe both masculine and feminine nouns without changing form:

• renda extra
• salário extra

In the plural, you will often see extras:

• rendas extras
• horas extras

So in this sentence, extra stays the same because it is singular.

Because fixa has to agree with despesa, which is feminine singular.

• despesa = feminine singular
• therefore fixa = feminine singular

Compare:

• despesa fixa
• custo fixo
• despesas fixas

This kind of adjective agreement is a basic and very important feature of Portuguese grammar.

Here cobre comes from the verb cobrir, and it means something like:

• covers
• is enough to cover
• pays for

So the idea is that the extra income is sufficient to pay that fixed expense.

This is a very common use of cobrir in financial contexts:

• A renda cobre os custos.
• O salário não cobre as contas.

Because the subject is a renda extra, which is third person singular.

The verb cobrir in the present tense goes like this:

• eu cubro
• você/ele/ela cobre
• nós cobrimos
• vocês/eles/elas cobrem

Since a renda extra is equivalent to ela, the correct form is cobre.

Because cobrir takes a direct object in this meaning.

So:

• cobrir algo = to cover something

That is why you say:

• A renda extra cobre a despesa fixa.

Not:

• cobre à despesa
• cobre para a despesa

Those would be wrong in this structure.

It can be understood either way, depending on context.

Most naturally, it sounds like a specific situation:

• the extra income covers the fixed expense

But Portuguese also uses definite articles in broader, more general statements more often than English does. So without more context, the sentence is grammatically fine and natural.

In normal speech, that sounds incomplete or unnatural.

Portuguese usually wants the articles here:

• A renda extra cobre a despesa fixa.

Without articles, it might sound like a headline, note, or very compressed style, but not like a normal full sentence.

They are close, but not always identical in tone.

• despesa fixa is a very common financial/accounting expression for fixed expense
• gasto fixo also exists and is understandable, but despesa fixa often sounds a bit more formal or technical

So in a budgeting or finance context, despesa fixa is an especially natural choice.

A rough Brazilian pronunciation would be:

a HEN-da EHS-tra KOH-bree a jees-PEH-zah FEEK-sah

A few useful notes:

• the r in renda is usually a throaty h-like sound in Brazil
• cobre sounds like KOH-bree
• in much of Brazil, de before e can sound like jee, so despesa may begin something like jees-
• fixa sounds like FEEK-sah

Pronunciation varies by region, but that approximation will help you recognize it.

It would become:

As rendas extras cobrem as despesas fixas.

Here is what changes:

• a → as
• renda → rendas
• extra → extras
• cobre → cobrem
• despesa fixa → despesas fixas

This is a good example of how articles, nouns, adjectives, and verbs all change together in Portuguese.