El sistema muestra ingresos como si fueran gastos.

Because sistema is a masculine noun in Spanish: el sistema.

This can feel surprising because nouns ending in -a are often feminine, but there are exceptions. Many words of Greek origin ending in -ma are masculine, for example:

• el sistema
• el problema
• el tema
• el programa

So the article el is correct here.

Here ingresos means income, revenue, or money coming in.

It is a very common accounting or financial term. In a business or software context, ingresos usually refers to money received, as opposed to gastos, which are expenses.

So in this sentence:

• ingresos = money coming in
• gastos = money going out

Spanish often leaves out the article when talking about things in a general or category-like way.

So muestra ingresos means shows income / shows revenues, not necessarily shows the income.

If you added an article, the meaning would become more specific:

• muestra ingresos = shows income/revenue in general
• muestra los ingresos = shows the income / the revenues

Both can be possible depending on context, but without the article it sounds more like a general accounting category.

Como si means as if.

It introduces an unreal, imagined, or apparently false comparison. In this sentence, the idea is that the system is presenting one thing in the way it would present another thing:

• como si fueran gastos = as if they were expenses

This structure is very common in Spanish:

• Habla como si lo supiera todo. = He talks as if he knew everything.
• Me mira como si estuviera enfadado. = He looks at me as if he were angry.

Because como si is normally followed by the imperfect subjunctive.

That is a key grammar rule:

• como si + imperfect subjunctive

So:

• como si fueran = as if they were

Not:

• como si son
• como si eran

The subjunctive is used because the comparison is not presented as a real fact. The sentence does not say that the income really is expenses; it says the system shows it as if it were.

Because it refers back to ingresos, which is plural.

The hidden subject of fueran is basically ellos, meaning the incomes / the revenues:

• ingresos → plural
• therefore fueran → plural

Compare:

• El sistema muestra el ingreso como si fuera un gasto.
  Singular: ingreso → fuera
• El sistema muestra ingresos como si fueran gastos.
  Plural: ingresos → fueran

Because gastos is being used as a category, not as a specific set of expenses.

So como si fueran gastos means as if they were expenses, in the general sense.

If you said como si fueran los gastos, that would usually suggest some specific expenses already known in the context, which is a different meaning.

This is similar to English, where we often say:

• treated as expenses not necessarily
• treated as the expenses

Yes. Fueran and fuesen are both valid imperfect subjunctive forms of ser.

So both are grammatical:

• como si fueran gastos
• como si fuesen gastos

In modern everyday Spanish, fueran is usually more common, but fuesen is also correct and may sound slightly more formal or literary in some contexts.

Because the subject is el sistema, which is singular.

The structure is:

• El sistema = subject
• muestra = singular verb, from mostrar
• ingresos = direct object

So the verb agrees with el sistema, not with ingresos.

Compare:

• El sistema muestra ingresos. = The system shows income.
• Los sistemas muestran ingresos. = The systems show income.

Yes. Muestra is the third person singular present of mostrar.

Here it means something like:

• shows
• displays
• presents

In a technical or software context, mostrar is very often used for what appears on a screen or in a report.

So El sistema muestra ingresos... is a very natural way to say that the system displays or reports income in a certain way.

Yes, that is also possible, but it changes the nuance slightly.

• muestra ingresos = shows income/revenue as a category
• muestra los ingresos = shows the income / the revenues, probably a specific set already known

So the version with los is more definite and specific. The version without los is more general and often sounds natural in accounting or software descriptions.