Si el parabrisas se llena de polvo, casi no veo la carretera.

Because this is a real or possible condition: If the windshield gets filled with dust, I can hardly see the road.

In Spanish, after si for real conditions, you normally use the indicative, not the subjunctive:

• Si el parabrisas se llena de polvo... = if the windshield gets filled with dust...

A learner might be tempted to say si se llene, but that is not correct here.

The sentence also uses the present tense in both parts because it sounds like a general fact or a habitual situation. Spanish often does this where English also uses the present:

• Si llueve, no salgo.
• Si el parabrisas se llena de polvo, casi no veo la carretera.

Because llenar and llenarse are used differently.

• llenar algo = to fill something
  
  • El polvo llena el parabrisas = the dust fills the windshield
• llenarse de algo = to fill up with something / to become full of something
  
  • El parabrisas se llena de polvo = the windshield gets full of dust

So here, the windshield is undergoing the change, and Spanish uses the pronominal form llenarse.

This se does not really mean it fills itself in a literal English sense. It is just part of the way Spanish expresses becoming full.

Because llenarse normally goes with de.

A very common pattern in Spanish is:

• llenarse de agua
• llenarse de humo
• llenarse de gente
• llenarse de polvo

So se llena de polvo is the natural combination.

Using con is not the usual choice after llenarse. A learner should treat llenarse de as a fixed pattern.

Because Spanish often omits subject pronouns when they are not needed.

The verb form veo already tells you the subject is I. So:

• veo = I see

That means yo is unnecessary unless you want emphasis, contrast, or clarity:

• Yo casi no veo la carretera, pero tú sí. = I can hardly see the road, but you can.

In the original sentence, yo would sound possible, but not necessary.

It means I can hardly see or I almost can't see.

The structure casi no + verb is very common in Spanish for expressing hardly / barely:

• Casi no duermo = I hardly sleep
• Casi no como pan = I hardly eat bread
• Casi no veo la carretera = I can hardly see the road

So even though the words look like almost not see, the natural English meaning is hardly see.

Because casi no works together as a natural unit meaning hardly or almost not.

So the normal order is:

• casi no veo
• casi no entiendo
• casi no salgo

No casi veo is not the natural way to say this.

This is one of those patterns that is best learned as a chunk:

• casi no + verb

Because Spanish often uses the simple verb where English prefers can + verb.

So:

• Casi no veo la carretera

very naturally means:

• I can hardly see the road

Spanish does not always need an extra verb like poder to express ability in these cases.

You could say:

• Casi no puedo ver la carretera

and it is correct, but it sounds a bit more explicit. The original version is more natural and direct.

Spanish uses the definite article more often than English does.

Here, la carretera refers to the road in the driving situation being described. It is the normal choice.

Spanish often keeps the article with nouns that are understood from the context:

• Abre la puerta
• Me duele la cabeza
• No veo la carretera

So even if English sometimes feels less specific, Spanish usually still uses el / la / los / las.

They are similar, but not identical.

• se llena de polvo = it gets filled with dust / it becomes dusty
• está lleno de polvo = it is full of dust

So the difference is:

• se llena focuses on the change
• está lleno focuses on the state

Compare:

• Si el parabrisas se llena de polvo, casi no veo la carretera.
  If the windshield gets covered/fills up with dust, I can hardly see the road.

• Si el parabrisas está lleno de polvo, casi no veo la carretera.
  If the windshield is full of dust, I can hardly see the road.

Both are possible, but the nuance changes.

Yes, absolutely.

That version is correct and natural. It just shifts the focus slightly:

• se llena de polvo = when it becomes dusty
• está lleno de polvo = when it is already dusty

So the original sentence is a bit more dynamic, while está lleno is more descriptive.

Because noun gender in Spanish is not always predictable from the ending.

Parabrisas is a masculine noun, so you say:

• el parabrisas

This is a good reminder that endings can help, but they are not perfect rules. With many nouns, especially compound nouns, you simply need to learn the noun together with its article.

So it is best to memorize it as:

• el parabrisas = the windshield

Because the words are related but not identical.

• carretera = road, highway, main road
• calle = street, usually in a town or city
• camino = path, track, way, or road in a broader sense

In a driving context, la carretera sounds very natural if you mean the road ahead. If the context were specifically urban, la calle might be more likely.

So la carretera fits the sentence well.