La rubsako estas plena.

La is the definite article in Esperanto, equivalent to the in English.

So La rubsako means the trash bag or the garbage bag, not just a trash bag.

A few useful points:

• Esperanto has only one definite article: la
• It does not change for gender, number, or case
• There is no separate word for a/an in ordinary Esperanto

So:

• rubsako = a trash bag / trash bag
• la rubsako = the trash bag

Esperanto very often builds new words by combining roots into a single compound word.

Here:

• rub' = trash, garbage
• sako = bag

So rubsako literally means trash-bag.

This is very normal in Esperanto. The last part is usually the main idea, and the earlier part modifies it. So a rubsako is a kind of sako.

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

Esperanto verbs do not change depending on the subject, so:

• mi estas = I am
• vi estas = you are
• li estas = he is
• ŝi estas = she is
• ĝi estas = it is
• ili estas = they are

So La rubsako estas plena literally follows the same structure as English: The trash bag is full.

The ending -a marks an adjective in Esperanto.

So plena is an adjective meaning full.

In this sentence, plena describes rubsako, so it has the adjective ending:

• plena = full

This is one of the most important Esperanto patterns:

• nouns end in -o
• adjectives end in -a
• adverbs end in -e
• infinitive verbs end in -i

Because the sentence needs an adjective, not a noun or an adverb.

• plena = full, as an adjective
• pleno = fullness, a noun
• plene = fully, an adverb

Here, full describes the bag, so the adjective form is required:

• La rubsako estas plena. = The trash bag is full.

If you used the others, the meaning or grammar would change:

• pleno would mean something like fullness
• plene would mean fully, which does not fit after estas here

Because plena is not the direct object here.

The -n ending is usually used for:

• direct objects
• motion toward something in some expressions

In La rubsako estas plena, the phrase after estas is a predicate adjective, not a direct object. It describes the subject.

So:

• La rubsako = subject
• estas = is
• plena = adjective describing the subject

That is why plena stays without -n.

Yes, and it does.

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

Here:

• rubsako is singular
• it is not accusative
• so the adjective is also singular and not accusative: plena

If the noun changed, the adjective would change too:

• La rubsako estas plena. = The trash bag is full.
• La rubsakoj estas plenaj. = The trash bags are full.
• Mi vidas la plenan rubsakon. = I see the full trash bag.

So the agreement rule is working normally here.

It does not have to. Plena can stand by itself if the context makes the meaning clear.

So La rubsako estas plena simply means The trash bag is full.

If you want to say what it is full of, you usually add de:

• La rubsako estas plena de rubo. = The trash bag is full of trash.

Both are correct:

• without de + noun if the contents are obvious or unimportant
• with de + noun if you want to specify the contents

Yes, Esperanto word order is fairly flexible, but the normal neutral order here is:

• La rubsako estas plena.

That is the most straightforward way to say it.

You might also see other orders for emphasis, such as:

• Plena estas la rubsako.

But that sounds more marked or literary. For everyday use, La rubsako estas plena is the best choice.

No. They are related, but they mean different things.

• rubo = trash, garbage
• sako = bag
• rubsako = trash bag / garbage bag

So rubo is the waste itself, while rubsako is the bag used for it.

That is why you could say:

• La rubsako estas plena de rubo.

This literally means the trash bag is full of trash.

Yes, if the context already makes it clear which kind of bag you mean.

• La sako estas plena. = The bag is full.
• La rubsako estas plena. = The trash bag is full.

Using rubsako is more specific. Using sako is more general.

So both are correct, but rubsako gives more information.