Imagu novan dekoracion.

Because -u is the Esperanto verb ending for the imperative/volitive mood.

In other words, it is used for:

• commands
• requests
• invitations
• exhortations

So imagu means something like imagine!

Compare:

• imagi = to imagine
• imagas = imagines / is imagining
• imagis = imagined
• imagu = imagine!

In Esperanto, commands usually leave out the subject because it is understood.

So:

• Imagu novan dekoracion.
• Vi imagu novan dekoracion.

Both are grammatical, but the first is more natural in most situations.

English does the same thing:

• Imagine a new decoration. not usually
• You imagine a new decoration.

Because dekoracion is the direct object, and in Esperanto direct objects usually take -n.

Since nova describes dekoracio, the adjective must agree with the noun. That means it matches:

• number
• case

So:

• dekoracio = decoration
• dekoracion = decoration as a direct object
• nova dekoracio = a new decoration
• novan dekoracion = a new decoration as a direct object

This agreement is a very important Esperanto rule.

Esperanto has no indefinite article.

So English a or an is usually simply not translated.

That means:

• dekoracio can mean a decoration or just decoration, depending on context
• la dekoracio means the decoration

In this sentence, novan dekoracion naturally means a new decoration.

Because that is the most common and neutral order in Esperanto: adjective + noun.

So:

• novan dekoracion = the normal order

But Esperanto word order is fairly flexible, so you can also put the adjective after the noun:

• dekoracion novan

That is still grammatical, but it sounds more marked, poetic, or emphatic. Most learners should use novan dekoracion unless they have a special reason not to.

Yes. Esperanto allows fairly flexible word order because the -n ending helps show what the direct object is.

So all of these are possible:

• Imagu novan dekoracion.
• Novan dekoracion imagu.

The second version gives extra emphasis to novan dekoracion.

Still, the first version is the most straightforward and natural for a learner.

A rough English-friendly guide is:

• imagu → ee-MA-goo
• novan → NO-vahn
• dekoracion → de-ko-ra-TSI-on

A few useful pronunciation points:

• Stress in Esperanto is always on the second-to-last syllable
• c is pronounced ts
• Each vowel is pronounced clearly

So the stressed syllables are:

• iMAgu
• NOvan
• de-ko-ra-CI-on

The dictionary forms are:

• imagi = to imagine
• nova = new
• dekoracio = decoration

In the sentence, they change form because of grammar:

• imagi → imagu for a command/request
• nova → novan to agree with the direct object
• dekoracio → dekoracion because it is the direct object

This is very typical Esperanto word-building:

• -i = infinitive verb
• -u = command/volitive verb
• -a = adjective
• -o = noun
• -n = accusative/direct object marker

No. That would be ungrammatical in standard Esperanto.

The problem is that the direct object needs -n, and the adjective must agree with it.

So the correct form is:

• Imagu novan dekoracion.

Not:

• Imagu nova dekoracio.

If you leave off the -n endings, the sentence no longer shows the object correctly.

Usually it is very close, yes. It refers to a decoration or decorative item.

But like many similar-looking words, exact usage can depend on context. In many cases, dekoracio works well for:

• decoration
• ornament
• decorative element

So in this sentence, it is a perfectly normal word choice.