La glaso estas plena de akvo.

La is the definite article, meaning the.

So:

• la glaso = the glass
• glaso = a glass or just glass in a general sense

In this sentence, la glaso points to a specific glass.

In Esperanto, nouns end in -o.

So:

• glaso = glass
• akvo = water

This is one of the core regular patterns of Esperanto grammar.

Adjectives in Esperanto end in -a.

So:

• plena = full

Because plena describes glaso, it is an adjective.

The -n ending is usually used for the direct object or for motion toward something.

Here, plena is not a direct object. It is a predicate adjective, used after estas to describe the subject:

• La glaso = the subject
• estas plena = is full

So plena stays just plena, not plenan.

Esperanto usually expresses this with the verb esti (to be) plus an adjective.

So:

• estas = is
• plena = full

Together, estas plena literally means is full.

This works just like:

• La akvo estas malvarma = The water is cold.
• La glaso estas granda = The glass is big.

After plena (full), Esperanto normally uses de.

So:

• plena de akvo = full of water
• plena de sablo = full of sand
• plena de homoj = full of people

Using kun would suggest with, not the normal pattern for full of. For this idea, de is the correct choice.

Here de means of in the expression full of.

So:

• plena de akvo = full of water

More generally, de can have several uses in Esperanto, such as:

• possession or relation
• origin
• content

In this sentence, it introduces what fills the glass.

Because the sentence is talking about the substance inside the glass, not a particular previously identified body of water.

So:

• de akvo = of water
• de la akvo = of the water

If you said de la akvo, it would sound more like full of the water, referring to some specific water already known from context.

Yes. In Esperanto, adjectives agree with the nouns they describe in number and case.

Since glaso is:

• singular
• not accusative

the adjective is also:

• singular
• not accusative

So we get:

• la glaso estas plena

If the noun changed, the adjective would change too:

• La glasoj estas plenaj. = The glasses are full.
• Mi vidas la plenan glason. = I see the full glass.

Yes, Esperanto word order is fairly flexible, because the endings carry a lot of grammatical information.

For example, these are possible:

• La glaso estas plena de akvo.
• Plena de akvo estas la glaso.

But the first one is the most neutral and natural for learners.

Because Esperanto nouns keep their normal noun ending -o after prepositions.

So:

• akvo = water
• de akvo = of water

Unlike in some languages, the noun does not change form just because it comes after de.

The natural English translation is The glass is full of water.

Even though learners may think of with, Esperanto uses plena de for this idea. So when you see:

• plena de akvo

you should think:

• full of water

not full with water.