La carrera popular empieza en la línea de salida.

Not usually. In Spain, carrera popular often means a public race or community race that ordinary people can enter, such as a local fun run or road race.

So although popular can sometimes mean popular/famous, in this context it usually means something more like open to the public or mass-participation.

Spanish often uses the definite article (el, la, los, las) where English might not.

Here, la carrera popular means the race—probably a specific race already known from the context.

So:

• la carrera popular = the community race / the public race
• una carrera popular = a community race / a public race

In Spanish, most adjectives normally come after the noun.

So:

• carrera popular = popular/public race
• línea de salida = starting line

Putting the adjective after the noun is the most natural word order here.

Empieza is the third person singular present tense of empezar.

It matches la carrera, which is singular:

• la carrera empieza = the race starts

A few related forms:

• yo empiezo = I start
• tú empiezas = you start
• la carrera empieza = the race starts

Both empezar and comenzar can mean to start or to begin.

In this sentence, empieza is completely natural and common.
You could also say La carrera popular comienza en la línea de salida, and it would mean almost the same thing.

In everyday Spanish, empezar is often slightly more common and informal-neutral.

En here means at or in, depending on the context.

With locations, Spanish often uses en where English uses at:

• empieza en la línea de salida = starts at the starting line

So even though en often gets translated as in, here the natural English meaning is at the starting line.

Because de is often used in Spanish to connect two nouns, where English uses a compound noun or an adjective.

So:

• línea de salida = literally line of departure/start
• natural English: starting line

You usually do not need the article here.
De salida works like a set descriptive phrase.

Other similar examples:

• mesa de comedor = dining table
• zapatos de deporte = sports shoes

No. They are different:

• línea de salida = starting line
• meta = finish line

So this sentence is talking about where the race begins, not where it ends.

The accent mark shows the stress: LÍ-ne-a.

Without going too deep into spelling rules, the written accent tells you where the emphasis goes and helps show the correct pronunciation.

So:

• línea has three syllables: lí-ne-a

In standard Spain Spanish, it is pronounced roughly like em-PYE-tha.

A few points:

• ie is a diphthong, sounding like ye in one syllable
• z in most of Spain sounds like the th in thin
• stress falls on pie: em-PIE-za

So in Spain:

• empieza ≈ em-PYE-tha

In many Latin American accents, the z sounds like s, so it would be closer to em-PYE-sa.

Because the subject is la carrera, which is grammatically singular.

Spanish verbs agree with the grammatical subject, not with the number of people involved in the event.

So:

• La carrera empieza = singular subject, singular verb
• Las carreras empiezan = plural subject, plural verb

It is possible in some contexts, but it is less natural here.

The most natural version is:

• La carrera popular empieza en la línea de salida

Why?

• empezar en = start at a place
• desde means from, and it emphasizes origin more than location

So if you just want to say where the race starts, en la línea de salida is the best choice.

Yes, it can be.

In Spanish, the present tense is often used for scheduled future events, just like in English:

• The race starts at 9
• La carrera empieza a las 9

So this sentence could mean:

• the race generally starts there, or
• the race will start there in an upcoming event

Both are possible depending on context.