La dekoracio kaj la kostumoj estas pretaj.

Because dekoracio is singular and kostumoj is plural.

In Esperanto:

• nouns usually end in -o
• the plural is made by adding -j

So:

• dekoracio = decoration
• kostumo = costume
• kostumoj = costumes

The sentence has one decoration (or decoration as a single set/concept) and more than one costume.

La is the definite article, meaning the.

It does not change for gender, number, or case in ordinary use:

• la dekoracio = the decoration
• la kostumoj = the costumes

It is used twice because Esperanto normally places la before each noun phrase when both are definite. So la dekoracio kaj la kostumoj is the natural way to say the decoration and the costumes.

Because it describes both la dekoracio and la kostumoj together.

Esperanto adjectives agree with the noun they describe in number, and sometimes case. Here, the adjective preta becomes plural pretaj because the subject as a whole is plural:

• la dekoracio = one thing
• la kostumoj = several things
• together = more than one item, so plural

Compare:

• La dekoracio estas preta. = The decoration is ready.
• La kostumoj estas pretaj. = The costumes are ready.
• La dekoracio kaj la kostumoj estas pretaj. = The decoration and the costumes are ready.

Because Esperanto verbs do not change according to the subject.

Estas always means is/are/am in the present tense, depending on context.

So:

• Mi estas preta. = I am ready.
• Li estas preta. = He is ready.
• Ili estas pretaj. = They are ready.

Unlike English, the verb stays the same.

Kaj means and.

It joins words or phrases:

• la dekoracio kaj la kostumoj = the decoration and the costumes
• pano kaj fromaĝo = bread and cheese

It is a very common conjunction and does not change form.

Because nothing here is a direct object.

The sentence is built with:

• a subject: La dekoracio kaj la kostumoj
• a linking verb: estas
• an adjective: pretaj

The -n ending is usually used for a direct object, but after esti (to be), the word after it is not a direct object. It is describing the subject.

So:

• La knabo vidas la hundon. = The boy sees the dog.
  Here hundon gets -n because it is the direct object.
• La hundo estas preta. = The dog is ready.
  No -n, because preta describes the subject.

Because it describes nouns, not the manner of an action.

In Esperanto:

• adjectives end in -a
• adverbs usually end in -e

Here pretaj describes the decoration and the costumes, so it must be an adjective.

Compare:

• La kostumoj estas pretaj. = The costumes are ready.
  (pretaj describes the costumes)
• Ili laboras prete. would mean something like They work in a ready way, which is not the intended idea here.

So pretaj is the correct form.

Yes. That is also correct Esperanto.

Esperanto often allows either:

• esti + adjective: estas pretaj
• a verb form: pretas

So these are both natural:

• La dekoracio kaj la kostumoj estas pretaj.
• La dekoracio kaj la kostumoj pretas.

The version with estas pretaj may feel a bit more transparent to beginners because it clearly shows the adjective preta.

Yes, it is grammatically singular in this sentence.

Whether that is best in a real-world context depends on what is meant:

• la dekoracio = the decoration / the décor / the decorative setup
• la dekoracioj = the decorations

Esperanto sometimes uses a singular noun for something English might express as a mass or collective idea. If the intended meaning is a whole decorative arrangement, la dekoracio makes sense. If the meaning is several separate decorations, la dekoracioj would be more specific.

This sentence shows several very common Esperanto endings:

• -o = noun
  dekoracio
• -oj = plural noun
  kostumoj
• -a = adjective
  base form: preta
• -aj = plural adjective
  pretaj
• -as = present tense verb
  estas

Learning these endings helps you understand a lot of Esperanto very quickly.

Yes, to a degree.

The normal order is:

• subject + verb + complement

So:

• La dekoracio kaj la kostumoj estas pretaj.

But Esperanto word order is more flexible than English because endings show grammatical roles. Still, this standard order is the clearest and most natural for learners.

For example, Pretaj estas la dekoracio kaj la kostumoj is possible, but it sounds more marked or stylistic.

Yes. In Esperanto, adjectives agree with the nouns they describe.

That means:

• singular noun → singular adjective
• plural noun → plural adjective
• accusative noun → accusative adjective too

Examples:

• la kostumo preta is not correct as a full noun phrase; it should be la preta kostumo if the adjective directly modifies the noun
• La kostumo estas preta. = The costume is ready.
• La kostumoj estas pretaj. = The costumes are ready.
• Mi vidas la pretajn kostumojn. = I see the ready costumes.

So pretaj is plural because the subject is plural overall.

Because in this sentence, pretaj is not directly attached to the noun phrase. It comes after estas and works as a predicate adjective.

Compare the two patterns:

1. Attributive adjective: directly before or after a noun inside the noun phrase

   • la pretaj kostumoj = the ready costumes

2. Predicate adjective: after estas, describing the subject

   • La kostumoj estas pretaj. = The costumes are ready.

Here we have the second pattern, so pretaj comes after estas.