Mi ekkriis, kiam mi vidis fumon el la najbara apartamento.

Ek- is a very common Esperanto prefix that means to start doing something or to do something suddenly.

So:

• krii = to shout, cry out
• ekkrii = to suddenly cry out / to start shouting
• ekkriis = suddenly cried out / started shouting

In this sentence, Mi ekkriis gives the sense of a sudden reaction.

Because it is made from:

• ek-
  • krii

When the prefix ek- is added to a root that already begins with k, both consonants stay:

• ek-
  • krii → ekkrii

That is normal in Esperanto. The spelling reflects the parts of the word clearly.

The verb ending -is marks the past tense in Esperanto.

So:

• ekkrii = to cry out suddenly
• ekkriis = cried out suddenly

and

• vidi = to see
• vidis = saw

Esperanto verb endings are very regular:

• -as = present
• -is = past
• -os = future
• -us = conditional
• -u = command / wish
• -i = infinitive

Kiam means when.

In this sentence, it introduces a time clause:

• Mi ekkriis, kiam mi vidis fumon...
• I cried out when I saw smoke...

So kiam connects the main event with the moment it happened.

The -n ending marks the direct object.

Here, fumon is what was seen:

• mi vidis fumon = I saw smoke

Compare:

• fumo = smoke
• fumon = smoke, as the direct object of the verb

Esperanto uses -n to show which noun receives the action, so word order can stay flexible.

Esperanto uses la only for the. There is no separate word for a or an.

So:

• fumon can mean smoke or some smoke
• la fumon would mean the smoke, referring to specific smoke already known from context

In this sentence, plain fumon sounds natural because it means smoke in a general, newly noticed sense.

El means out of, from inside, or coming from.

So:

• fumo el la najbara apartamento = smoke coming out of the neighboring apartment

This is more specific than just saying de.
El emphasizes that the smoke is coming from inside that place.

Najbara means neighboring, nearby, or next-door.

It comes from:

• najbaro = neighbor
• najbara = neighboring / neighbor’s / nearby

The -a ending marks an adjective.

So:

• la najbara apartamento = the neighboring apartment / the next-door apartment

Because each word has its own job:

• la = the
• najbara = neighboring
• apartamento = apartment

Together:

• la najbara apartamento = the neighboring apartment

This works much like English adjective order, except Esperanto adjectives always take the -a ending.

Apartamento is simply the Esperanto word for apartment.

It has:

• the root apartament-
• the noun ending -o

So:

• apartamento = apartment

Many Esperanto words are built this way: a root plus a regular ending.

Because each clause normally states its own subject clearly.

The sentence has two clauses:

1. Mi ekkriis
2. kiam mi vidis fumon...

Even though the subject is the same person, Esperanto usually keeps the subject pronoun if it belongs to a new clause. This makes the sentence clear and natural.

Yes, it is normal and appropriate here.

Esperanto often uses a comma before a subordinate clause, especially one introduced by words like:

• kiam = when
• ke = that
• ĉar = because
• se = if

So:

• Mi ekkriis, kiam mi vidis fumon...

is correctly punctuated.

Yes, Esperanto word order is fairly flexible, because endings show grammatical roles.

For example, these are possible:

• Mi ekkriis, kiam mi vidis fumon el la najbara apartamento.
• Kiam mi vidis fumon el la najbara apartamento, mi ekkriis.

Both mean essentially the same thing. The second version puts more emphasis on the time clause first.

However, the original order is very natural.

Vidis fumon is a simple, natural way to say saw smoke.

• vidi = to see
• rimarki = to notice

Both could work in some contexts, but vidis is more basic and straightforward.

Also:

• vidis fumon = saw smoke
• vidis la fumon = saw the smoke

Using la would suggest the smoke is already specific or already known. Without la, it sounds like the speaker suddenly noticed smoke for the first time, which fits the situation well.