La capacidad testamentaria afecta a la disposición testamentaria.

Testamentaria is an adjective meaning relating to a will or testamentary.

So:

• capacidad testamentaria = testamentary capacity
• disposición testamentaria = testamentary disposition / provision made in a will

It appears twice because it is describing two different legal nouns: capacidad and disposición.

Both capacidad and disposición are feminine singular nouns, so they take la.

• la capacidad
• la disposición

In Spanish, articles are used more often than in English, especially in formal or general statements. So even where English might sometimes omit the, Spanish often keeps it.

In Spanish, adjectives usually come after the noun they describe.

So:

• capacidad testamentaria
• disposición testamentaria

This is the normal order for a classification-type adjective, especially in formal or technical language. It is basically saying the capacity related to wills and the disposition related to wills.

Putting testamentaria before the noun would sound unusual here.

Afecta is the third person singular present tense of afectar.

So it means:

• it affects

The subject of the sentence is La capacidad testamentaria, so the verb is singular:

• La capacidad testamentaria afecta...

If the subject were plural, the verb would change:

• Las capacidades testamentarias afectan...

Because afectar is commonly used with a before the thing or person affected:

• afectar a alguien
• afectar a algo

So:

• afecta a la disposición testamentaria

That a is part of the normal construction of the verb. It does not mean this is an indirect object in the English sense; it is just how afectar is used in Spanish.

Not exactly.

• testamento = a will
• disposición testamentaria = a provision, clause, or act of disposition made in a will

So disposición testamentaria is more specific and more technical. It refers to what is laid down in the will, not the whole will itself.

Spanish legal language commonly uses testamentario / testamentaria to mean related to a will.

Because capacidad and disposición are feminine nouns, the adjective must agree with them:

• testamentario with a masculine noun
• testamentaria with a feminine noun

Examples:

• el albacea testamentario
• la capacidad testamentaria

Yes. In Spanish, adjectives agree with the nouns they describe.

Here:

• capacidad is feminine singular → testamentaria
• disposición is feminine singular → testamentaria

If the nouns were plural, the adjective would also be plural:

• las disposiciones testamentarias

The normal and most natural order is:

• La capacidad testamentaria afecta a la disposición testamentaria.

You could change the order for emphasis, but it would sound more marked or less neutral. In legal or formal writing, the straightforward order is usually preferred.

So for a learner, this sentence is best understood as:

• subject: La capacidad testamentaria
• verb: afecta
• complement: a la disposición testamentaria

No, this is formal legal Spanish.

Words like:

• capacidad testamentaria
• disposición testamentaria

belong to legal vocabulary, especially inheritance and wills. A native speaker would understand the grammar easily, but the phrasing itself is specialized rather than conversational.

A useful approximation in Spain Spanish is:

• disposición → dee-spoh-see-THYON
• testamentaria → tes-tah-men-TA-ria

A couple of key points:

• ción is stressed at the end: disposiCIÓN
• In most of Spain, ci is pronounced with a th sound, so ción sounds like thyón
• testamentaria is stressed on ta: tes-ta-men-TA-ria

Spanish often has more than one way to express the same idea.

For example:

• la capacidad testamentaria
• la capacidad de testar

Both can be possible in legal contexts, but they are not identical in style.

• capacidad testamentaria sounds more technical and compact
• capacidad de testar is a little more transparent to learners because it uses the verb testar = to make a will

So this sentence uses the more formal nominal style that is common in legal writing.