El brillo afecta a la saturación.

Because Spanish nouns have grammatical gender. Brillo is masculine, so it takes el. Saturación is feminine, so it takes la.

Spanish also uses the definite article more often than English, especially with general, abstract, or technical ideas. So el brillo and la saturación are very natural here, even if English might sometimes just say brightness and saturation without articles.

Because the subject is el brillo, which is singular.

• el brillo afecta = the brightness affects
• los brillos afectan = the brightness levels / shines affect

The verb has to agree with the subject, not with the noun after a.

With afectar, Spanish very often uses the pattern afectar a algo/alguien to mark what is being affected.

So in:

El brillo afecta a la saturación

the thing being affected is la saturación.

That a does not mean to in a simple word-for-word way. It is just part of the normal structure used with this verb in sentences like this.

Because only a + el becomes al.

• a + el = al
• a + la = a la
• a + los = a los
• a + las = a las

So a la saturación stays exactly as it is.

Yes. Afectar is a regular -ar verb.

Its present tense forms are:

• yo afecto
• tú afectas
• él / ella / usted afecta
• nosotros afectamos
• vosotros afectáis
• ellos / ellas / ustedes afectan

In your sentence, afecta is the él/ella/usted form, because the subject is el brillo.

Because it is a feminine noun: la saturación.

A useful pattern is that many nouns ending in -ción are feminine:

• la saturación
• la información
• la solución
• la configuración

So when you see -ción, feminine gender is a very good guess.

The written accent shows where the stress falls: sa-tu-ra-CIÓN.

Without the accent, Spanish stress rules would suggest a different pronunciation. The accent tells you clearly that the last syllable is stressed.

This is also very common in words ending in -ción, such as:

• información
• edición
• resolución

A rough English guide would be BREE-yo.

A few details:

• br is pronounced together, with a tapped r
• i sounds like the ee in see
• ll in most of Spain sounds like y in yes
• the stress is on bri

So: BRI-yo.

Depending on the region, the ll sound can vary slightly, but BREE-yo is a good practical guide for most learners.

A rough guide for most of Spain is sa-too-ra-THYON.

Important points:

• satu- = sa-too-
• the stress is on the last syllable: -ción
• in most of Spain, c before i is pronounced like th in thin, so -ción sounds like -thyon
• in Latin America, it would usually sound more like -syon

So for Spain, sa-tu-ra-THYON is a helpful approximation.

No, that would sound wrong in normal Spanish.

Spanish usually needs the articles here:

• el brillo
• la saturación

So the natural sentence is:

El brillo afecta a la saturación.

Leaving out the articles would sound incomplete or unnatural.

Yes, but the original order is the most straightforward and natural:

El brillo afecta a la saturación.

That is a standard subject + verb + object structure.

You can change it for emphasis, but it may sound more marked or less neutral. For example:

• A la saturación la afecta el brillo.
  This is possible, but it puts extra focus on la saturación.

For most situations, especially as a learner, the original version is the best one to use.

Yes, but it is usually less direct and often less natural than the active sentence.

Possible passive-style versions are:

• La saturación es afectada por el brillo.
• La saturación se ve afectada por el brillo.

These are grammatical, but in many contexts Spanish prefers the active form:

El brillo afecta a la saturación.

That version is shorter, clearer, and very natural.