La akvotubo estas sub la planko.

Akvotubo is a compound word:

• akvo = water
• tubo = tube, pipe

Together, akvotubo means water pipe.

Esperanto very often builds vocabulary this way: you combine smaller meaningful parts into one word. Usually the main idea comes last, so tubo is the basic noun, and akvo tells you what kind of tube it is.

So:

• akvotubo = a tube/pipe for water
• literally, something like water-tube

Because there are two separate nouns, and each one is definite:

• la akvotubo = the water pipe
• la planko = the floor

In Esperanto, la is the definite article the, and it does not change for gender, number, or case in ordinary use. If both nouns are definite, each one normally gets its own la.

Because sub la planko is a prepositional phrase meaning under the floor.

• sub = under, below
• la planko = the floor

So estas sub la planko means is under the floor.

By contrast, sube is an adverb meaning below, down below, or underneath in a more general sense. It does not directly take a noun after it.

So:

• La akvotubo estas sub la planko. = The water pipe is under the floor.
• La akvotubo estas sube. = The water pipe is below/down below.

Because nothing here is a direct object, and the location expression does not require -n.

In Esperanto, -n is commonly used for:

• the direct object
• sometimes direction toward something

This sentence is just describing a state or location:

• La akvotubo = subject
• estas = is
• sub la planko = location

So no accusative is needed.

Compare:

• La akvotubo estas sub la planko. = The water pipe is under the floor.
• Mi vidas la akvotubon. = I see the water pipe.
  Here akvotubon gets -n because it is the direct object.

Estas is the present tense form of esti, which means to be.

• esti = to be
• estas = is / am / are

Esperanto verbs do not change according to the subject. That means the same form is used with I, you, he, she, it, we, they.

Examples:

• Mi estas = I am
• La akvotubo estas = the water pipe is
• Ili estas = they are

This is much simpler than English, where am / is / are are different forms.

The normal word order here is:

• La akvotubo = subject
• estas = verb
• sub la planko = complement/location

So it follows a very familiar pattern for English speakers.

Yes, Esperanto word order is somewhat flexible, so other orders can be possible for emphasis, but this version is the most neutral and natural for a learner to use.

For example, Sub la planko estas la akvotubo could be used in a context where you want to emphasize the location first, but La akvotubo estas sub la planko is the straightforward default sentence.

Sub specifically means under or below.

That makes it the natural choice when one thing is physically lower than another thing.

Some related prepositions are:

• sur = on
• super = above, over
• en = in
• apud = beside
• sub = under

So if the pipe is beneath the floor, sub la planko is exactly what you want.

Yes. Planko means floor, especially the floor surface inside a building.

If you wanted ground or earth, you would usually use a different word, such as tero.

So:

• sub la planko = under the floor
• not generally under the ground

That distinction matters, because the Esperanto word here points to a building floor, not the earth outside.

Esperanto spelling is very regular, so each letter has a consistent sound.

A helpful approximate pronunciation is:

• akvotubo ≈ ahk-vo-TOO-bo
• planko ≈ PLAHN-ko
• estas ≈ ES-tahs
• sub ≈ soob

Stress in Esperanto normally falls on the second-to-last syllable:

• ak-vo-TU-bo
• PLAN-ko
• ES-tas

That regular stress pattern is one of the nice features of Esperanto pronunciation.

You usually would not say that if you mean the water pipe is under the floor.

In this sentence, la helps show that we are talking about a specific pipe and a specific floor:

• la akvotubo = the water pipe
• la planko = the floor

If you remove la, the sentence sounds more general or less natural in this context.

Esperanto has no indefinite article like English a/an, so:

• akvotubo can mean a water pipe or water pipe depending on context
• la akvotubo clearly means the water pipe

Not usually.

• akvotubo means a water pipe: a pipe intended for carrying water
• tubo de akvo would more literally suggest a tube of water, which sounds like the tube itself is made of water or filled with water in a less natural way

In Esperanto, compounds like akvotubo are often the normal way to express this kind of everyday idea. English also does something similar with water pipe rather than pipe of water.