Los datos fiscales incluyen retenciones e ingresos a cuenta.

Los is the masculine plural definite article, meaning the.

It is used because datos is:

• plural → so the article must be plural
• masculine → so the article is los, not las

So:

• el dato = the piece of data / the item of information
• los datos = the data

Datos fiscales is a noun + adjective combination:

• datos = data / tax information / details
• fiscales = fiscal, tax-related

In Spanish, adjectives usually come after the noun, unlike in English.

So:

• datos fiscales = tax data or tax details

Also notice agreement:

• datos is masculine plural
• fiscales is also plural to match it

Because the subject is los datos fiscales, which is third person plural.

The verb incluir = to include

Present tense:

• incluye = he/she/it includes
• incluyen = they include

Since los datos fiscales = the tax details / the tax data, Spanish treats that as plural here, so the verb must also be plural:

• Los datos fiscales incluyen... = The tax data include...

The basic form, or infinitive, is incluir.

Incluyen is the present indicative, third person plural:

• ellos/ellas incluyen
• los datos incluyen

A quick mini-pattern:

• yo incluyo
• tú incluyes
• él/ella incluye
• nosotros incluimos
• vosotros incluís
• ellos/ellas incluyen

This is a very common Spanish spelling and pronunciation rule.

Normally, and is y.
But before a word that begins with an i sound, Spanish usually changes y to e to avoid an awkward repetition of sounds.

So:

• retenciones y pagos
• but retenciones e ingresos

Because ingresos begins with the vowel sound i, y becomes e.

This also happens in other examples:

• padres e hijos
• España e Italia

Retenciones is the plural of retención.

In tax and payroll contexts in Spain, retención usually means a withholding: money kept back in advance, often for tax purposes.

So:

• una retención = a withholding
• retenciones = withholdings

This is a technical financial/tax word, and it is very common in Spanish administrative language.

This is a set tax expression.

• ingresos = payments / income / sums paid, depending on context
• a cuenta = on account / in advance / as a prepayment

In tax language, ingresos a cuenta usually refers to advance payments made on account, often amounts paid in advance toward a tax obligation.

Even if you know ingresos as income, here the whole expression should be understood as a fixed administrative phrase, not translated word by word too mechanically.

After verbs like incluir, Spanish often lists things without repeating articles when speaking in a general sense.

So:

• incluyen retenciones e ingresos a cuenta

means:

• include withholdings and advance payments

You could sometimes see articles in other contexts, but here the zero article sounds natural and general, like listing categories or components.

Yes. In this sentence, datos is clearly a plural count noun in Spanish.

Spanish commonly uses:

• un dato = one piece of data / one item of information
• los datos = the data / the details

So the grammar stays plural:

• Los datos fiscales incluyen...

Even if English sometimes says the data is in informal use, Spanish here uses straightforward plural agreement.

Very literally, it is:

The tax data include withholdings and payments on account.

That helps show the structure:

• Los datos fiscales = the tax data
• incluyen = include
• retenciones = withholdings
• e = and
• ingresos a cuenta = payments on account / advance payments

This kind of literal breakdown is useful for grammar, even if a smoother English translation might sound slightly different.