La akvumilo estas plena.

La is the definite article in Esperanto, meaning the.

So la akvumilo means the watering can / the watering device, not just a watering can.

A few useful points:

• Esperanto has only one article: la
• It does not change for gender, number, or case
• Esperanto normally has no indefinite article, so akvumilo by itself can mean a watering can or just watering can, depending on context

Akvumilo is built from smaller parts:

• akv- = water
• -um- = a suffix with less predictable meaning, here part of akvumi, to water
• -il- = tool or instrument
• -o = noun ending

So akvumilo literally means something like a tool for watering.

Depending on context, that could mean:

• a watering can
• a sprinkler
• another watering device

If the translation you were shown says watering can, that is a very natural interpretation.

In Esperanto, nouns end in -o.

So:

• akvumilo = a noun
• without -o, it would not be a normal standalone noun form

This is one of the most regular parts of Esperanto grammar:

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

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

So:

• mi estas = I am
• vi estas = you are
• ĝi estas = it is

In your sentence, La akvumilo estas plena, estas means is.

Esperanto verbs do not change according to the subject, so estas stays the same with all persons and numbers.

Because plena is an adjective, and adjectives in Esperanto end in -a.

Here, plena describes akvumilo:

• akvumilo = watering can/device
• plena = full

So the structure is:

• La akvumilo = the watering can
• estas plena = is full

Because this is a predicate adjective after estas.

In Esperanto, the -n ending is usually used for:

• the direct object
• motion toward something in some cases

But after esti (to be), the adjective is not a direct object. It is describing the subject.

So:

• La akvumilo estas plena. = correct
• plenan would be wrong here

Also, adjectives agree with the noun they describe in number and case. Since akvumilo is singular and not accusative, plena is also singular and not accusative.

Yes, plenas exists and means is full or is filled.

So La akvumilo plenas is possible.

However, for a beginner, La akvumilo estas plena is often easier to understand because it uses the very common pattern:

• noun + esti
  • adjective

That pattern works exactly like English is full.

This sentence uses the most neutral word order:

• La akvumilo = subject
• estas = verb
• plena = complement

So it is a standard subject–verb–complement pattern.

Esperanto word order is fairly flexible, but this version is the most straightforward and natural in ordinary usage.

A simple pronunciation guide:

• La = lah
• akvumilo = ahk-voo-MEE-lo
• estas = ES-tahs
• plena = PLEH-nah

Stress in Esperanto is always on the second-to-last syllable:

• akvuMIlo
• EStas
• PLEna

That fixed stress rule makes pronunciation much more predictable than in English.

Usually plena means full in the ordinary sense.

In this sentence, it most naturally means the watering can is full, probably with water or some other liquid.

If you want to say what it is full of, Esperanto can add that information, for example:

• La akvumilo estas plena de akvo. = The watering can is full of water.

So plena works much like English full, and context tells you what is inside unless you specify it.

Not always.

Because the word literally means watering tool/device, it can be a broader term than English watering can.

Depending on context, it might refer to:

• a watering can
• a sprinkler
• some other watering implement

So the exact English translation depends on the situation, even though the Esperanto word formation is very clear.