Mi iam loĝis en tiu apartamento.

Here iam means at some time, once, or sometime. In this sentence, it gives the idea I once lived in that apartment.

A very common beginner confusion is iam vs. jam:

• iam = ever / sometime / once
• jam = already

So Mi iam loĝis... is I once lived..., not I already lived...

The ending -is marks the past tense in Esperanto.

So:

• loĝi = to live
• loĝas = lives / am living
• loĝis = lived
• loĝos = will live

In Esperanto, tense is shown very regularly by the verb ending, so loĝis clearly tells you the action happened in the past.

Yes, here loĝi means to live in the sense of to reside somewhere.

So Mi iam loĝis en tiu apartamento means that the speaker resided there at some point in the past.

It does not mean to be alive in a general sense. It is specifically about where someone lives.

Because this sentence describes a location, not movement.

• en tiu apartamento = in that apartment (already there; location)
• en tiun apartamenton = into that apartment (movement toward the inside)

Esperanto often uses -n after a preposition when there is motion toward a place. Here there is no movement; it just says where the speaker lived, so no -n is needed.

Tiu means that or that one.

Before a noun, it works like that in English:

• tiu apartamento = that apartment

If you wanted this apartment, you would usually say ĉi tiu apartamento.

Because tiu already makes the noun specific.

In English, we say that apartment, not that the apartment. Esperanto works the same way here:

• tiu apartamento = that apartment

Also, Esperanto has no indefinite article a/an. So nouns are either:

• bare: apartamento = an apartment / apartment
• definite with la: la apartamento = the apartment
• specified in another way: tiu apartamento = that apartment

Because tiu is not a regular adjective like bela or granda. It is a demonstrative correlative, and it has its own form.

Even so, it can modify a noun just like an English determiner:

• tiu apartamento = that apartment

It can still take some endings when needed:

• tiuj apartamentoj = those apartments
• tiun apartamenton = that apartment as a direct object
• tiujn apartamentojn = those apartments as direct objects

So it does agree for plural and accusative, but it does not become tiua.

Yes, you could.

Esperanto word order is fairly flexible. The original sentence

Mi iam loĝis en tiu apartamento

is a neutral, natural order. But

Iam mi loĝis en tiu apartamento

is also correct and puts a bit more emphasis on iam.

The meaning stays basically the same.

Yes, it can, depending on context.

The Esperanto past tense -is is broader than English forms. Loĝis can correspond to:

• lived
• used to live
• sometimes was living

Here, because of iam, the sentence strongly suggests at one time I lived there, so I once lived in that apartment or I used to live in that apartment are both natural interpretations.

A rough pronunciation is LOH-jis, with the stress on the first syllable.

A few details:

• ĝ sounds like the j in jam
• o is a pure vowel, like o in many continental European languages
• i sounds like ee
• stress in Esperanto is always on the second-to-last syllable

So loĝis is stressed LO-ĝis.

Yes, but the meaning changes slightly.

• en tiu apartamento = in that apartment
• en la apartamento = in the apartment

Tiu points to a particular apartment, as if the speaker or listener already has that one in mind. La simply marks it as definite, without the same pointing sense.

So if you specifically mean that one, tiu is the better choice.