Mia oficejo estas sur la tria etaĝo, en ĉambro numero dek du.

Mia means my. It is a possessive adjective, so it modifies a noun just like an adjective does.

• mi = I
• mia = my

So mi oficejo would be ungrammatical.

As for la, Esperanto normally does not use la before a possessive adjective when it directly modifies a noun. So mia oficejo is the normal way to say my office.

Also, possessive adjectives can agree like other adjectives when needed:

• mia oficejo = my office
• miaj oficejoj = my offices
• mian oficejon = my office, as a direct object

Here the form is simple because the noun is singular and not accusative.

Oficejo means office in the sense of a room, workplace, or office space.

The ending -ejo is a very common Esperanto suffix meaning place used for something. So:

• ofico = office, post, function, official role
• oficejo = office as a place

That is why mia oficejo means my office as a room or workplace, not my official position.

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

One useful feature of Esperanto is that verbs do not change according to the subject:

• mi estas = I am
• vi estas = you are
• li estas = he is
• ili estas = they are

So in this sentence, estas simply means is.

Because the sentence gives two different kinds of location.

• sur la tria etaĝo = on the third floor
• en ĉambro numero dek du = in room number twelve

Sur means on, and it is the natural preposition for being on a floor of a building.

En means in or inside, so it is used for being inside a room.

So the sentence first tells you the floor, then the room.

Because a specific floor is meant: the third floor.

In Esperanto, ordinal expressions often take la when they refer to a particular, identifiable item:

• la unua tago = the first day
• la dua ĉapitro = the second chapter
• la tria etaĝo = the third floor

Without la, tria etaĝo sounds more like a descriptive phrase than a specific, definite one. In this sentence, la is the most natural choice.

Tria is an ordinal adjective, meaning third.

Esperanto forms ordinals by adding -a to the number word:

• unu = one → unua = first
• du = two → dua = second
• tri = three → tria = third

Because tria is an adjective, it describes etaĝo.

Because the sentence describes location, not movement toward a place.

• Mia oficejo estas... tells where the office is
• nothing is moving anywhere

So you get:

• sur la tria etaĝo
• en ĉambro numero dek du

with no accusative -n.

If movement were involved, you might see a different structure, for example:

• Mi iras al la tria etaĝo = I am going to the third floor
• Mi eniras ĉambron numero dek du = I enter room number twelve

A very common beginner rule is:

• place where something is → usually no -n
• direction or movement → often a different form appears

Because this kind of phrase often works like a label: in room number twelve.

In Esperanto, when a noun is followed by a number label or identifying designation, the article is often omitted quite naturally:

• ĉambro numero dek du
• paĝo numero kvin
• linio numero tri

Using la is also possible in some contexts, especially if you want to stress that it is a specific known room:

• en la ĉambro numero dek du

But the version in the sentence is perfectly natural.

Numero dek du explicitly means number twelve.

This is useful because it makes it clear that 12 is the room’s identifying number, not its position in a sequence.

So:

• ĉambro numero dek du = room number 12

That is slightly different from saying the twelfth room, which would focus on order rather than the number printed on the door.

In real life, you may also see shortened forms such as n-ro 12 or simply ĉambro 12.

Dek du means twelve, literally ten two.

Esperanto numbers are very regular:

• dek = 10
• dek unu = 11
• dek du = 12
• dek tri = 13
• dudek = 20
• dudek du = 22

So numero dek du is simply number twelve.

Etaĝo means a floor or storey of a building.

A useful related word is:

• teretaĝo = ground floor

This matters because English speakers do not always count floors the same way in different countries. Esperanto itself gives you the word etaĝo, but if there is any risk of confusion, speakers can clarify by mentioning teretaĝo or by explaining the building’s numbering system.

So la tria etaĝo is grammatically straightforward, but real-world interpretation can still depend on local convention.

A few sounds here are especially worth noticing:

• ĉ in ĉambro sounds like ch in church
• ĝ in etaĝo sounds like j in judge
• j in oficejo sounds like y in yes
• c in oficejo sounds like ts

So approximate pronunciations are:

• oficejo ≈ o-fi-TSEY-o
• etaĝo ≈ e-TA-jo
• ĉambro ≈ CHAM-bro

Esperanto also has fixed stress: the stress falls on the second-to-last syllable.

So:

• o-fi-CEJ-o
• e-TA-ĝo
• ĈAM-bro