El astigmatismo afecta a la visión.

Because nouns ending in -ismo are almost always masculine in Spanish. For example:

• el turismo
• el realismo
• el alcoholismo

So it is el astigmatismo.

Afecta is the third-person singular present indicative of the verb afectar.

That matches the subject el astigmatismo, which is singular:

• El astigmatismo afecta...
• Los problemas afectan...

So the verb changes depending on the subject.

Because afectar is very commonly used as afectar a alguien/algo. In other words, Spanish often treats a as part of the normal pattern of the verb.

So it is very natural to say:

• afecta a la visión
• afecta al corazón
• afecta a muchas personas

For a learner, it is useful to remember the pattern afectar a.

Not exactly.

The personal a is used before a person as a direct object, but here la visión is not a person. In this sentence, the a is there because of how afectar is commonly constructed: afectar a.

So it is better to think of it as part of the verb pattern, not as the personal a.

Because Spanish only contracts these two combinations:

• a + el = al
• de + el = del

But a + la does not contract, so it stays a la.

That is why the sentence has a la visión, not a shortened form.

Spanish often uses the definite article with abstract nouns and general body-related concepts where English might not.

So la visión is completely natural in Spanish, even though English often just says vision without the.

The accent mark shows where the stress goes.

A Spanish word ending in -n is normally stressed on the second-to-last syllable. But visión is stressed on the last syllable, so it needs an accent mark:

• vi-SIÓN

Without the accent mark, you would expect a different stress pattern.

A rough English-style approximation is:

el ahs-tee-gmah-TEES-moh ah-FEK-tah ah lah bee-SYON

A few helpful notes:

• astigmatismo is stressed on -tis-
• afecta is stressed on -fec-
• visión is stressed on -sión
• In most Spanish accents, b and v sound very similar, so visión begins with a sound close to English b

Sometimes, yes, but the nuance is a little different.

• la vista often means eyesight or the sense of sight in everyday language
• la visión is also correct and sounds especially natural in medical or technical contexts

So in this sentence, la visión is an excellent choice.

Because el astigmatismo is the subject of the sentence, and the verb afecta agrees with it in the singular.

So the structure is:

• El astigmatismo = subject
• afecta = verb
• a la visión = what is affected

In other words, astigmatism is the cause, and vision is what receives the effect.