Antaŭ la vojaĝo mi plenigas la botelon per akvo.

Antaŭ is a preposition meaning before. It is used directly before a noun phrase:

• antaŭ la vojaĝo = before the trip

Antaŭe is an adverb meaning beforehand / previously / earlier. It does not directly take a noun in the same way.

So:

• Antaŭ la vojaĝo = before the trip
• Antaŭe = beforehand, earlier

In this sentence, you need the preposition because la vojaĝo follows it.

Because antaŭ is a preposition, and nouns after a preposition normally do not take -n.

So:

• antaŭ la vojaĝo = before the trip

The -n ending is mainly used for the direct object, and sometimes for direction, but not here.

Plenigi means to fill.

It comes from:

• plena = full
• -ig- = to make something become

So:

• plenigi = to make full = to fill

That means:

• mi plenigas la botelon = I fill the bottle

By contrast:

• mi estas plena = I am full
• pleniĝi = to become full

So plenigas is exactly the right verb when the subject causes something else to become full.

Because la botelon is the direct object of plenigas.

Ask: What do I fill?
Answer: the bottle

So the object gets -n:

• mi plenigas la botelon

This is one of the most basic uses of the Esperanto accusative.

Because per usually means by means of / using / with.

In this sentence, the water is what you use to fill the bottle:

• plenigi la botelon per akvo = to fill the bottle with water

Using de here would sound wrong, because de usually means of, from, by in other senses, not the material or substance used to fill something in this pattern.

So after plenigi, per is the normal choice for what you fill it with.

Here per means something like using or by means of.

Compare:

• Mi skribas per krajono. = I write with a pencil.
• Mi plenigas la botelon per akvo. = I fill the bottle with water.

In both cases, per introduces the means or substance involved in the action.

Esperanto present tense -as is used for present actions, habitual actions, general statements, and sometimes narration.

So mi plenigas la botelon can mean things like:

• I fill the bottle
• I am filling the bottle
• I fill the bottle (as a regular thing)

Without more context, Esperanto -as is broader than English simple present in some cases.

If you wanted a clear past action, you would use -is:

• Antaŭ la vojaĝo mi plenigis la botelon per akvo. = Before the trip, I filled the bottle with water.

Yes, Esperanto word order is fairly flexible because the grammar endings show the roles clearly.

This sentence could also be:

• Mi plenigas la botelon per akvo antaŭ la vojaĝo.

That still means the same thing.

However, placing Antaŭ la vojaĝo at the beginning gives it emphasis and sets the time first, which sounds natural.

La is the definite article, meaning the.

So:

• la vojaĝo = the trip
• la botelon = the bottle

Esperanto has only one definite article: la. It does not change for gender, number, or case, although the noun itself may change:

• la botelo
• la botelon
• la boteloj
• la botelojn

Sometimes Esperanto can omit la, but that changes the meaning.

• antaŭ la vojaĝo = before the trip (a specific trip)
• antaŭ vojaĝo = before a trip / before travel

In your sentence, la vojaĝo suggests a particular trip known from context, so la is appropriate.

Because akvo is not the direct object here.

The direct object is la botelon — that is the thing being filled.

Akvo is inside the prepositional phrase per akvo, and nouns after a preposition normally do not take -n.

So:

• la botelon = object → -n
• per akvo = after a preposition → no -n

Yes.

• plenigas = fills, makes something full
• pleniĝas = becomes full, gets full

So:

• Mi plenigas la botelon per akvo. = I fill the bottle with water.
• La botelo pleniĝas per akvo. = The bottle is getting full with water.

The first sentence focuses on the person doing the action. The second focuses on the bottle undergoing the change.

They are related, but one is a noun and one is a verb:

• vojaĝo = a trip, a journey
• vojaĝi = to travel

In the sentence:

• Antaŭ la vojaĝo = before the trip

If you wanted to use the verb, you would need a different structure, for example:

• Antaŭ ol mi vojaĝas... or more naturally Antaŭ ol vojaĝi...

So vojaĝo is correct because the sentence refers to the trip as a thing.

A rough English-friendly pronunciation would be:

ahn-TAU la voy-AH-zho mee plen-EE-gahs la bo-TEL-on pehr AK-vo

A few helpful points:

• ŭ in antaŭ sounds like a short w glide
• j sounds like English y
• ĝ sounds like the j in jam
• stress is always on the second-to-last syllable:
  • anTAŬ
  • voJAĝo
  • pleNIgas
  • boTElon
  • AKvo

Yes, it is natural and grammatically correct.

It uses very standard Esperanto patterns:

• time phrase first: Antaŭ la vojaĝo
• subject: mi
• transitive verb: plenigas
• direct object with -n: la botelon
• prepositional phrase showing what is used: per akvo

So it is a good model sentence for learning several common Esperanto structures at once.