La presbicia afecta a la vista.

In Spanish, nouns have grammatical gender, and both presbicia and vista are feminine nouns, so they take the article la.

• la presbicia = presbyopia / age-related farsightedness
• la vista = sight / eyesight

The article has to match the noun’s gender and number, so:

• la presbicia afecta...
• la vista

This is grammatical gender, not biological gender.

Because the verb afectar normally uses the pattern afectar a + noun/pronoun when you say what is being affected.

So:

• afectar a la vista = to affect the eyesight
• afectar a los ojos = to affect the eyes
• afectar a muchas personas = to affect many people

That a is part of how the verb is commonly constructed here. It is not the same as the English to.

Afecta is the third-person singular present tense of afectar.

The subject is la presbicia, which is singular, so the verb must also be singular:

• La presbicia afecta... = Presbyopia affects...

Other present-tense forms are:

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

Because the subject is singular: la presbicia.

In Spanish, the verb agrees with the subject:

• La presbicia afecta a la vista.
  Singular subject → singular verb

If the subject were plural, then you would use afectan:

• Algunas enfermedades afectan a la vista.

Here, la vista means sight or eyesight.

Spanish often uses la vista in health-related contexts to talk about a person’s ability to see:

• tener buena vista = to have good eyesight
• afectar a la vista = to affect eyesight

Related words:

• la vista = sight, eyesight
• la visión = vision, sometimes a bit more technical or abstract
• los ojos = the eyes (the physical organs)

So:

• afectar a la vista focuses on sight/eyesight
• afectar a los ojos focuses more on the eyes themselves

In standard Spanish, especially in careful or neutral usage, afecta a la vista is the safer and more natural choice.

You may sometimes encounter afectar used without a, especially in some varieties or contexts, but for a learner, it is best to remember:

• afectar a algo / a alguien

So this sentence is best learned as:

• La presbicia afecta a la vista.

Spanish often uses the definite article where English does not, especially with body parts and related concepts.

So Spanish commonly says:

• la vista
• los ojos
• la cabeza
• las manos

Even when English would often leave out the, Spanish keeps it:

• Me duelen los ojos. = My eyes hurt.
• La presbicia afecta a la vista. = Presbyopia affects eyesight.

This is very normal Spanish usage.

The normal and most natural order is:

• La presbicia afecta a la vista.

Spanish word order is somewhat flexible, but unusual orders often sound literary, emphatic, or unnatural in everyday speech.

For a learner, the standard order to use is:

• subject + verb + complement

So it is best to keep:

• La presbicia afecta a la vista.

In standard Peninsular Spanish, it is pronounced approximately:

• pres-BI-thia

More roughly:

• pres as in press
• bi stressed: BEE
• cia in much of Spain sounds like thia because c before i is pronounced like the th in thin

So in Spain:

• presbicia ≈ pres-BEE-thya

In many parts of Latin America, it would sound more like:

• pres-BEE-sya

The stress is on bi: presBIcia.

It is a correct and standard medical word, but in everyday conversation people may also explain it more simply rather than use the technical term.

For example, someone might say:

• Tengo presbicia.
• Necesito gafas para leer.
• Con la edad, me cuesta ver de cerca.

So presbicia is perfectly correct, but it can sound a bit medical or technical depending on context.

No. Afectar is a very common verb and can be used in many contexts, not just medical ones.

Examples:

• La contaminación afecta al medio ambiente.
• El ruido afecta al sueño.
• La crisis afecta a muchas familias.
• La presbicia afecta a la vista.

It generally means to affect, to have an impact on, or to influence negatively or noticeably.

Yes. You could replace a la vista with an indirect object pronoun only in some rephrased contexts, but in this exact sentence Spanish usually keeps the full phrase because afectar a commonly names the thing affected directly.

For example:

• La presbicia le afecta. = Presbyopia affects him/her.

But if you want to say specifically eyesight, it is clearer and more natural to keep:

• La presbicia afecta a la vista.

So learners should probably not try to shorten this sentence too early. The full version is the most straightforward.

The plural would be presbicias, but in practice the word is usually treated as an uncountable medical condition, so the singular is much more common.

Most often you will hear:

• la presbicia

rather than:

• las presbicias

So in this sentence, the singular is exactly what you would expect.