Post kiam mia fratino lernis stiri pli trankvile, ŝi ne plu timis la grandajn kamionojn en la urbo.

Post kiam means after in the sense of after the time when.

Esperanto often uses:

• post = after
• kiam = when

So post kiam mia fratino lernis stiri pli trankvile literally means after when my sister learned to drive more calmly, which is the normal Esperanto way to introduce a time clause.

You would not usually use just post by itself before a whole clause like this. Post works well before a noun phrase, but before a full clause Esperanto commonly uses post kiam.

Because the whole sentence is talking about past events.

• lernis = learned
• timis = was afraid / feared

The idea is:

1. First, she learned to drive more calmly.
2. After that, she was no longer afraid of the big trucks.

Esperanto past tense is very straightforward:

• -is = past
• -as = present
• -os = future

So both verbs are in the past because both actions are being described from a past point of view.

Because after lerni when you mean learn to do something, the second verb stays in the infinitive.

• lerni = to learn
• stiri = to drive

So:

• lernis stiri = learned to drive

This works much like English learned to drive. In Esperanto, the infinitive ends in -i, so stiri is the basic verb form.

Other similar patterns:

• komencis kuri = began to run
• volis manĝi = wanted to eat
• provis dormi = tried to sleep

Stiri means to steer or to drive a vehicle.

In this sentence, it clearly means to drive. So lernis stiri means learned to drive.

A learner may notice that English separates steer and drive, but Esperanto stiri can cover the idea of operating and directing a vehicle.

Because it describes how she drove, so Esperanto uses an adverb.

• trankvila = calm, शांत, peaceful as an adjective
• trankvile = calmly as an adverb

Here it modifies the verb stiri:

• stiri trankvile = to drive calmly

Compare:

• trankvila ŝoforo = a calm driver
• ŝi veturis trankvile = she traveled calmly

A very useful rule:

• -a = adjective
• -e = adverb

Pli means more.

So:

• trankvile = calmly
• pli trankvile = more calmly

Even without saying than before, Esperanto can still use pli if the comparison is understood from context. Here the idea is that she learned to drive in a calmer way than earlier.

If you wanted to state the comparison explicitly, you could use ol:

• pli trankvile ol antaŭe = more calmly than before

Ne plu means no longer or not anymore.

So:

• ŝi ne plu timis = she was no longer afraid

This is a very common Esperanto expression:

• plu by itself has the sense of further / any longer / anymore
• ne plu = no longer

Examples:

• Mi ne plu loĝas tie. = I no longer live there.
• Li ne plu laboras ĉi tie. = He doesn’t work here anymore.

That is a normal Esperanto word order:

• ŝi = she
• ne = not
• plu = any longer / anymore
• timis = feared / was afraid of

So the sentence is built in a fairly natural way: subject + negation + adverb + verb.

Esperanto word order is flexible, but this order is clear and common.
The important thing is that ne negates the verb phrase, and plu adds the idea of any longer.

Because it is the direct object of timis.

She feared the big trucks, so the big trucks receives the action of the verb. In Esperanto, direct objects take -n.

• kamiono = truck
• kamionoj = trucks
• kamionojn = trucks as a direct object

This is the accusative ending.

So:

• ŝi timis la kamionojn = she feared the trucks

Because adjectives must agree with the nouns they describe.

The noun is:

• kamionojn

It is:

• plural: -j
• accusative: -n

So the adjective must match:

• granda kamiono = a big truck
• grandaj kamionoj = big trucks
• grandajn kamionojn = big trucks as a direct object

This agreement is one of the core features of Esperanto grammar.

La is the definite article, meaning the.

So la grandajn kamionojn means the big trucks, not just big trucks in a general sense.

Esperanto has only one article:

• la = the

There is no separate word for a/an. If Esperanto leaves out la, the meaning is often indefinite or more general.

So:

• ŝi timis grandajn kamionojn = she was afraid of big trucks
• ŝi timis la grandajn kamionojn = she was afraid of the big trucks

In context, la may refer to the big trucks in that city environment as a known group.

Because this phrase shows location, not movement toward a place.

• en la urbo = in the city
• en la urbon = into the city

Esperanto uses -n after a preposition in some cases to show direction or motion toward something. But here the sentence is not saying that the trucks moved into the city. It just says where they were: in the city.

So the location form without -n is correct.

Because Post kiam mia fratino lernis stiri pli trankvile is a subordinate time clause, and it is followed by the main clause:

• subordinate clause: Post kiam mia fratino lernis stiri pli trankvile
• main clause: ŝi ne plu timis la grandajn kamionojn en la urbo

Using a comma here is normal and helps mark the boundary between the two parts of the sentence.

Yes. Esperanto allows fairly flexible word order, especially because endings show grammatical roles clearly.

For example, you could also say:

Ŝi ne plu timis la grandajn kamionojn en la urbo, post kiam mia fratino lernis stiri pli trankvile.

That still means essentially the same thing.
However, the original order is very natural because it first gives the earlier event and then the result.

Because Esperanto normally states the subject explicitly.

In the first clause you have mia fratino, and in the second clause you have ŝi referring back to her:

• mia fratino ... ŝi ...

That is normal and clear. Esperanto does not usually drop subject pronouns the way some languages do. Repeating the subject with ŝi makes the second clause complete and easy to understand.

Both are good translations.

• timi can mean to fear
• in many contexts, natural English may prefer to be afraid of

So:

• ŝi timis la grandajn kamionojn
  can be understood as either
• she feared the big trucks or
• she was afraid of the big trucks

The Esperanto grammar is the same either way: timi takes a direct object, which is why kamionojn has -n.