La afiŝo estas sur la pordo de la biblioteko.

Because Esperanto uses la for each noun that is definite or specific.

So in this sentence:

• la afiŝo = the poster
• la pordo = the door
• la biblioteko = the library

Unlike in English, you do not normally let one the cover several later nouns unless they are part of the same noun phrase. Each definite noun here gets its own la.

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

So:

• mi estas = I am
• ĝi estas = it is
• la afiŝo estas = the poster is

Esperanto verbs do not change according to the subject. The form estas is used for I am, you are, he is, they are, and so on, whenever the tense is present.

Sur means on or on top of.

So sur la pordo means on the door.

This is a very direct match with English:

• sur la tablo = on the table
• sur la muro = on the wall
• sur la pordo = on the door

Since the sentence describes location, sur is the natural preposition.

Because pordo is not a direct object here. It comes after the preposition sur.

In Esperanto, nouns after prepositions usually do not take -n when they show a static location:

• sur la pordo = on the door
• en la biblioteko = in the library

You would use -n with sur if you wanted to show movement onto something:

• Mi metas la afiŝon sur la pordon. = I put the poster onto the door.

But in your sentence, the poster is already there, so sur la pordo is correct.

Here, de means something like of.

So la pordo de la biblioteko means the door of the library, or more naturally in English, the library’s door.

In Esperanto, de is very common for showing possession, belonging, or association:

• la libro de Maria = Maria’s book
• la ĉambro de la instruisto = the teacher’s room
• la pordo de la biblioteko = the library’s door

Yes, de can also mean from.

For example:

• Mi venas de la biblioteko. = I come from the library.

You tell the meaning from the structure and context.

In la pordo de la biblioteko, two nouns are linked together, so de is understood as of or belonging to, not from.

So here it does not mean the door from the library. It means the door of the library.

Both patterns are possible in Esperanto, but they are not always used in exactly the same way.

• la pordo de la biblioteko = the door of the library
• la biblioteka pordo = literally the library door

The de phrase is often the most straightforward way to say that something belongs to or is part of something specific.

Biblioteka pordo can work, but it can sound a little more descriptive, like a library door or a door associated with a library, rather than focusing on one specific library.

So la pordo de la biblioteko is a very natural, clear choice.

Yes. Esperanto word order is more flexible than English word order.

This sentence has a neutral, basic order:

• La afiŝo estas sur la pordo de la biblioteko.

You could also say:

• Sur la pordo de la biblioteko estas la afiŝo.

That still means essentially the same thing, but it puts more focus on the location first.

Even though Esperanto allows flexibility, the original version is probably the most natural one for a beginner to learn first.

A simple pronunciation guide is:

la a-FI-sho ES-tas sur la POR-do de la bib-li-o-TE-ko

A few useful points:

• ŝ sounds like sh
• stress in Esperanto normally falls on the second-to-last syllable
• each letter is pronounced clearly

So:

• afiŝo has stress on FI
• pordo has stress on POR
• biblioteko has stress on TE

Esperanto has a definite article, la, but it does not have an indefinite article like English a or an.

So:

• afiŝo can mean a poster
• la afiŝo means the poster

In this sentence, the speaker is talking about specific things, so la is used. If you removed la, the meaning would become less specific and would sound more like a poster is on a door of a library, depending on context.