La luanto loĝas sur la tria etaĝo.

Luanto comes from:

• lui = to rent
• -ant- = a person doing the action
• -o = noun ending

So luanto means the person who rents the place — that is, the tenant.

A useful contrast:

• luanto = the renter / tenant
• luiganto = the person who rents it out

Esperanto often separates two ideas that English combines in live:

• loĝi = to reside / live somewhere
• vivi = to live / be alive

Since this sentence is about where someone resides, loĝas is the natural verb.

Also, -as shows the present tense, so loĝas means is living / lives.

Because they belong to two different noun phrases:

• la luanto = the tenant
• la tria etaĝo = the third floor

Esperanto uses la for something definite or specific. Here, both the tenant and the floor are understood as specific.

Tri is the number three.

To make an ordinal number such as third, Esperanto adds -a:

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

So tria is the adjective meaning third.

Because they are different parts of speech:

• etaĝo is a noun, so it ends in -o
• tria is an adjective, so it ends in -a

This is one of the most regular features of Esperanto:

• nouns: -o
• adjectives: -a
• adverbs: -e

For floors of a building, Esperanto normally uses sur.

So:

• sur la tria etaĝo = on the third floor

This may feel slightly different from English logic, but it is the standard Esperanto way to talk about what level of a building something is on.

En usually means in / inside, so en la tria etaĝo is less natural here. If you were emphasizing being inside rooms on that level, other phrasing might be possible, but sur is the normal basic choice.

There is no direct object in this sentence.

• La luanto is the subject
• loĝas is the verb
• sur la tria etaĝo is a place expression

Also, after a preposition like sur, Esperanto normally uses no -n when you are talking about location.

Compare:

• sur la tria etaĝo = location
• sur la trian etaĝon = movement onto/up to that floor

In real usage, for movement people also often say al la tria etaĝo, depending on the exact meaning.

The letter ĝ sounds like the s in measure or the g in genre.

So roughly:

• loĝas ≈ LO-zhas
• etaĝo ≈ e-TA-zho

Esperanto stress always falls on the second-to-last syllable, so:

• LO-ĜAS
• e-TA-ĝo

Not automatically.

Etaĝo means a floor / storey of a building, but Esperanto itself does not force either the American or the British numbering system. In practice, speakers usually follow the local convention.

So depending on context, la tria etaĝo may line up differently with what an American English speaker or a British English speaker expects.