La fulmo ekbrilis super la rivero, sed la pluvo baldaŭ ĉesis.

The prefix ek- often shows the beginning of an action, often with a sudden or brief feel.

So:

• brilis = was shining / shone
• ekbrilis = began to shine / flashed

With fulmo (lightning), ekbrilis is very natural because lightning happens suddenly.

La is the definite article, meaning the.

In Esperanto, la is used when the speaker has a specific thing in mind. In this sentence, the speaker is talking about a particular lightning flash, a particular river, and the rain in that scene.

English sometimes treats weather words more loosely, but Esperanto can still use la naturally:

• la fulmo = the lightning
• la rivero = the river
• la pluvo = the rain

So this does not sound strange in Esperanto.

Because this phrase describes location, not motion toward something.

• super la rivero = above the river
• super la riveron would suggest movement in a direction, roughly to a position above the river

Here, the lightning flashed in a position above the river, so no -n is needed.

Super means above or over.

In this sentence, it tells you where the lightning flashed: over the river.

It is a preposition, so it introduces a prepositional phrase:

• super la rivero = over the river

Baldaŭ is an adverb meaning soon.

It modifies the verb ĉesis, telling you when the rain stopped.

Esperanto adverbs usually end in -e, but some very common adverb-like words, including baldaŭ, do not follow that pattern. You simply learn them as they are.

Because it modifies the verb and gives the sentence a natural rhythm.

Esperanto word order is fairly flexible, so you could also say:

• La pluvo ĉesis baldaŭ
• Baldaŭ la pluvo ĉesis

But la pluvo baldaŭ ĉesis is very natural and clear.

Ĉesi means to stop or to cease.

In this sentence:

• la pluvo ĉesis = the rain stopped

It can be used in different ways:

• La pluvo ĉesis. = The rain stopped.
• Li ĉesis paroli. = He stopped speaking.
• Ili ĉesigis la bruon. = They caused the noise to stop.

Notice that ĉesi itself often means something stops on its own, while ĉesigi means to make something stop.

Because -is is the Esperanto ending for the past tense.

So:

• ekbrilis = flashed / began to shine
• ĉesis = stopped

Esperanto tense endings are very regular:

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

These are two special Esperanto letters that often interest English speakers.

• ĉ is pronounced like ch in church
• ŭ is a short w-like glide, not a full vowel

So:

• ĉesis starts with a ch sound
• baldaŭ ends with something like ow in now, where ŭ helps form the diphthong

A rough guide:

• ĉesis ≈ CHEH-sis
• baldaŭ ≈ BAL-dow

Yes, it is normal and helpful.

Sed means but, and it connects two main clauses:

• La fulmo ekbrilis super la rivero
• sed la pluvo baldaŭ ĉesis

Using a comma before sed is standard punctuation in Esperanto, just as it often is before but in English.

Yes. Esperanto allows some flexibility in word order, as long as the meaning stays clear.

For example, these are possible:

• Super la rivero la fulmo ekbrilis, sed la pluvo baldaŭ ĉesis.
• La fulmo super la rivero ekbrilis, sed la pluvo baldaŭ ĉesis.
• La fulmo ekbrilis super la rivero, sed baldaŭ la pluvo ĉesis.

However, the original version is smooth, clear, and very natural.