La biciklo kostas dudek eŭrojn.

La is the definite article, meaning the. It shows that the speaker means a specific bicycle, not just any bicycle.

So:

• La biciklo = the bicycle
• Biciklo = a bicycle / bicycle in a more general or indefinite sense

Esperanto has la for the, but it does not have a separate word for a/an.

In Esperanto, nouns normally end in -o.

So:

• biciklo = bicycle
• domo = house
• hundo = dog

This makes it easy to recognize that biciklo is a noun.

The ending -as marks the present tense in Esperanto.

So:

• kostas = costs / is costing
• kostis = cost / did cost
• kostos = will cost
• kosti = to cost

In this sentence, kostas means costs or is worth in the present.

No. Esperanto verbs do not change according to person or number.

So all of these use the same present-tense ending -as:

• mi kostas
• vi kostas
• li kostas
• ili kostas

That means kostas stays the same whether the subject is singular or plural, first person or third person.

Dudek means twenty and is built from:

• du = two
• dek = ten

So dudek literally means two tens.

Numbers in Esperanto usually do not change form. So dudek stays dudek no matter what noun follows it.

Examples:

• dudek eŭroj = twenty euros
• dudek libroj = twenty books

The ending -j marks the plural.

So:

• eŭro = euro
• eŭroj = euros

Because the sentence says twenty euros, the noun has to be plural: eŭroj.

The -n is the accusative ending. Here it marks dudek eŭrojn as the direct object of kostas.

In Esperanto, with kosti, the thing being priced is the subject, and the amount is treated as the object:

• La biciklo kostas dudek eŭrojn. = The bicycle costs twenty euros.

So:

• La biciklo = subject
• kostas = verb
• dudek eŭrojn = object

That is why you get both endings together:

• eŭro = euro
• eŭroj = euros
• eŭrojn = euros, as a plural direct object

Because biciklo is the subject, not the object.

The bicycle is the thing that costs a certain amount. The amount of money is the object.

So in:

• La biciklo kostas dudek eŭrojn

the roles are:

• La biciklo = subject
• dudek eŭrojn = object

Only the object gets -n here.

Yes, you could, but the meaning changes.

• La biciklo kostas dudek eŭrojn = The bicycle costs twenty euros
• Biciklo kostas dudek eŭrojn = more like A bicycle costs twenty euros or Bicycles cost twenty euros, depending on context

So la makes the bicycle definite and specific.

Yes, Esperanto word order is fairly flexible, because endings help show what each word is doing.

The neutral, most ordinary order is:

• La biciklo kostas dudek eŭrojn.

But other orders are possible for emphasis, such as:

• Dudek eŭrojn kostas la biciklo.

That might sound more marked or emphatic, something like Twenty euros is what the bicycle costs.

Still, for normal learning and everyday use, subject + verb + object is the safest pattern.

A simple English-friendly approximation is:

• EH-oo-royn

A few details:

• eŭ sounds like a quick combination somewhat like ehw
• j in Esperanto sounds like English y
• oj sounds roughly like oy

So eŭrojn ends with a sound like royn.

If you want the whole sentence approximately:

• La biciklo kostas dudek eŭrojn
• lah bee-TSI-klo KOS-tahs DOO-dek EH-oo-royn

Yes. Eŭro is the normal Esperanto form of euro.

The letter ŭ is a real Esperanto letter, and it often appears in combinations like aŭ and eŭ.

So:

• eŭro = euro
• eŭroj = euros
• eŭrojn = euros, in the accusative plural

Even though the word comes from an international currency name, it follows normal Esperanto spelling and grammar.