El ciclista lleva casco porque la carretera está mojada.

Both can be correct, but llevar is very common in Spanish for things you wear or have on you.

• lleva casco = he is wearing a helmet / he has a helmet on
• usa casco = he uses a helmet

In this sentence, lleva casco sounds more natural because it describes what the cyclist is wearing.

Spanish often leaves out the indefinite article with clothing, accessories, and some everyday objects, especially after verbs like llevar.

So Spanish commonly says:

• lleva casco
• lleva gafas
• lleva chaqueta

Even though English often says a helmet, Spanish does not always need un here.

Spanish uses definite articles much more often than English. Here, El ciclista means the cyclist.

If you said just ciclista, it would usually sound incomplete in this kind of sentence unless it appeared in a different structure, such as:

• Es ciclista = He/She is a cyclist

But as the subject of a normal sentence, el ciclista is the standard form.

Because carretera is a feminine noun, so it takes the feminine article la.

• la carretera
• una carretera

A lot of nouns ending in -a are feminine, and carretera follows that common pattern.

Spanish uses estar for conditions or states, especially temporary ones. A road being wet is seen as a condition, so Spanish says:

• la carretera está mojada = the road is wet

Using ser here would sound wrong because mojada is not an essential, permanent characteristic of the road.

A useful shortcut:

• ser = what something is
• estar = how something is

Because adjectives usually agree with the noun they describe in gender and number.

Here, carretera is:

• feminine
• singular

So the adjective must also be feminine singular:

• carretera mojada

Compare:

• el camino mojado = the wet path
• la carretera mojada = the wet road
• las carreteras mojadas = the wet roads

These are different words:

• porque = because
• por qué = why

So in this sentence, porque introduces the reason:

• lleva casco porque la carretera está mojada

If you were asking a question, you would use por qué:

• ¿Por qué lleva casco? = Why is he wearing a helmet?

Ciclista can refer to either a male or female cyclist. The article tells you the gender in this sentence:

• el ciclista = the male cyclist
• la ciclista = the female cyclist

So here, el ciclista refers to a male cyclist.

Lleva is the third person singular present tense of llevar.

So it means:

• he wears
• she wears
• it wears
• he is wearing / she is wearing, depending on context

In Spanish, the present tense often covers both the simple present and the present continuous when the context is clear.

The accent distinguishes está (from estar) from esta (this, feminine).

• está = is
• esta = this

So:

• la carretera está mojada = the road is wet
• esta carretera = this road

The written accent is important because it changes the meaning.

Usually carretera means road and often a road connecting places, rather than a city street.

Some useful comparisons:

• carretera = road
• calle = street
• autopista / autovía = motorway / highway

So la carretera está mojada is best understood as the road is wet.

Yes, you could say that. Spanish allows flexible word order, especially when moving the reason clause to the front.

Both are correct:

• El ciclista lleva casco porque la carretera está mojada.
• Porque la carretera está mojada, el ciclista lleva casco.

The first version is more neutral and natural in most contexts. The second puts more emphasis on the reason.

Yes, it is grammatically possible, but it is less natural in a general statement like this.

• lleva casco = the normal, idiomatic way
• lleva un casco = possible, but it may sound more specific, as if you are emphasizing a helmet as an object

In everyday Spanish, with clothing and wearable items, dropping the article is very common.

The personal a is used before a specific human direct object, not before subjects or things.

Here:

• el ciclista is the subject
• casco is a thing, not a person
• la carretera is also a thing

So no personal a is needed.

Example with personal a:

• Veo al ciclista = I see the cyclist

But in your sentence, el ciclista is doing the action, so there is no personal a.

Yes, it sounds completely natural in Spain Spanish.

All the words and structures are standard:

• ciclista
• lleva casco
• porque
• la carretera está mojada

A speaker from Spain would understand and use this sentence naturally.