La biciklistino haltis ĉe la ruĝa semaforo.

Esperanto words are often built from smaller meaningful pieces:

• bicikl- = bicycle
• -ist- = a person associated with an activity, occupation, or practice
• -in- = female
• -o = noun ending

So biciklistino means a female cyclist.
The related form biciklisto is cyclist in general, and traditionally can also mean a male cyclist depending on context.

-is is the Esperanto past-tense verb ending.

So:

• haltas = stops / is stopping
• haltis = stopped
• haltos = will stop

The verb root is halt-, and Esperanto uses fixed verb endings, which makes tense very regular.

La is the definite article, meaning the.

So:

• la biciklistino = the cyclist
• la ruĝa semaforo = the red traffic light

Esperanto has only one definite article: la.
It does not change for gender, number, or case.

Also, Esperanto has no separate word for a/an, so an indefinite noun would just appear without an article.

Because -a is the adjective ending in Esperanto.

• ruĝa = red
• bela = beautiful
• granda = big

In Esperanto, adjectives usually agree with the noun in number and case. Here semaforo is singular and not accusative, so ruĝa stays in the basic form -a.

The -n ending is usually used for:

1. a direct object
2. direction toward something in some expressions

This sentence has neither.

• La biciklistino is the subject, not the object.
• haltis is an intransitive verb here, so it does not take a direct object.
• ĉe la ruĝa semaforo is a prepositional phrase, and the preposition ĉe does not require -n here.

So no word needs the accusative ending.

Ĉe usually means at, by, or near. It gives a location.

So ĉe la ruĝa semaforo means something like:

• at the red traffic light
• by the red traffic light

If you used al, that would suggest movement to or toward the traffic light. But the sentence is talking about where she stopped, not where she was heading at that moment.

Semaforo means traffic light or traffic signal.

In this sentence, la ruĝa semaforo most naturally refers to the traffic light when it is red, or the red signal at the light.

So even though English often says at the red light, Esperanto commonly uses semaforo here.

In Esperanto, the article goes before the whole noun phrase, not before each word.

So:

• la ruĝa semaforo = the red traffic light

You do not say la ruĝa la semaforo.

This works much like English, where we say the red traffic light, not the red the traffic light.

Esperanto word order is fairly flexible, because the endings show what each word is doing. The normal, neutral order here is:

• La biciklistino haltis ĉe la ruĝa semaforo.

But other orders are possible for emphasis, such as:

• Ĉe la ruĝa semaforo haltis la biciklistino.

That still makes sense, but it sounds more marked or stylistically different. For a learner, the original order is the safest and most natural choice.

A couple of sounds here may stand out to an English speaker:

• ĉ is like ch in church
• ĝ is like j in judge

So:

• ĉe sounds roughly like cheh
• ruĝa sounds roughly like roo-zha or roo-ja, depending on how carefully you hear the ĝ

A fuller rough pronunciation of the whole sentence is:

lah bee-tsee-klee-STEE-noh HAL-tis cheh lah ROO-zhah seh-mah-FO-roh

Remember that Esperanto stress normally falls on the second-to-last syllable.

Yes. Halti means to stop in the sense of come to a stop.

It is usually intransitive:

• La aŭto haltis. = The car stopped.
• La biciklistino haltis. = The cyclist stopped.

If you want to stop something in a causative sense, Esperanto often uses a different form such as haltigi:

• Li haltigis la aŭton. = He stopped the car.

So in your sentence, haltis is exactly the right form because the cyclist herself came to a stop.