La tubisto laboras antaŭ la masonisto.

Because Esperanto uses la for each definite noun phrase.

• la tubisto = the plumber
• la masonisto = the mason / bricklayer

Just like in English, if both people are specific and identifiable, both nouns can take the.

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

The ending -o marks a noun in Esperanto.

So:

• tubisto is a noun
• masonisto is a noun

This is one of the most basic Esperanto patterns:

• -o = noun
• -a = adjective
• -e = adverb
• -i = infinitive verb
• -as = present-tense verb

The suffix -ist- usually refers to a person զբաղնող a profession, occupation, or regular activity.

So these words are built like this:

• tub- + -ist- + -o → tubisto = a person who works with pipes, a plumber
• mason- + -ist- + -o → masonisto = a person who does masonry/brickwork

This suffix is very common in Esperanto. For example:

• pianisto = pianist
• verkisto = writer
• dentisto = dentist

Because -as is the present-tense verb ending in Esperanto.

• labori = to work
• laboras = works / is working

A very helpful feature of Esperanto is that verbs do not change according to the subject:

• mi laboras = I work
• vi laboras = you work
• li laboras = he works
• ili laboras = they work

The ending stays -as every time.

It can mean either, depending on context.

In Esperanto, the simple present often covers both:

• works
• is working

So La tubisto laboras antaŭ la masonisto could be understood as either a general statement or something happening now, depending on the situation.

Esperanto does have more explicit ways to stress an ongoing action, but in normal usage laboras is usually enough.

Here antaŭ means in front of.

But antaŭ can also mean before in a time sense. For example, it can refer to something happening earlier.

So antaŭ can express:

• place: in front of
• time: before

In this sentence, the meaning shown to the learner tells you which one is intended.

Because this sentence describes location, not movement toward a place.

• antaŭ la masonisto = in front of the mason

The plumber is working in that position. Nothing is moving into that position, so no accusative -n is needed.

A useful rule of thumb:

• place/location → usually no -n
• direction toward somewhere → often -n

So this sentence uses the normal location form.

Yes. Esperanto word order is fairly flexible.

The neutral order here is:

• La tubisto laboras antaŭ la masonisto.

But you could also say:

• Antaŭ la masonisto laboras la tubisto.

That version puts more focus on the location phrase.

Even though Esperanto allows flexibility, the original sentence is the most straightforward and natural for a beginner.

Because Esperanto has no indefinite article.

English distinguishes:

• a plumber
• the plumber

Esperanto only has a definite article, la. If you leave out la, the noun can often mean a/an or just a general noun, depending on context.

So:

• tubisto = a plumber / plumber
• la tubisto = the plumber

In your sentence, la shows that both people are specific.

You would add -j to the nouns.

• tubisto → tubistoj
• masonisto → masonistoj

So the plural version would be:

• La tubistoj laboras antaŭ la masonistoj.

The verb laboras does not change in the plural, which is another very convenient feature of Esperanto.