Kiam ni malfermis la pordon, malvarma aero eniris, kaj ni sentis densan fumon el la najbara apartamento.

Kiam means when and introduces a time clause. The whole part Kiam ni malfermis la pordon sets the time for what happens next.

So the structure is:

• Kiam ni malfermis la pordon = when we opened the door
• malvarma aero eniris = cold air entered
• kaj ni sentis densan fumon el la najbara apartamento = and we sensed dense smoke from the neighboring apartment

Putting the Kiam-clause first is very natural in Esperanto, just as in English.

The ending -is marks the past tense in Esperanto. So malfermis means opened or did open.

A useful point for English speakers: Esperanto past tense does not force the same aspect distinctions that English does. Depending on context, malfermis could correspond to:

• opened
• was opening
• had opened

But in this sentence, opened is clearly the intended reading.

The -n marks the direct object.

Here, la pordon is the thing being opened, so it gets the accusative ending:

• ni malfermis la pordon = we opened the door

This is one of the most basic uses of -n in Esperanto.

Because la pordon is the direct object, while malvarma aero is the subject.

Compare the roles:

• ni = subject of malfermis
• la pordon = object of malfermis
• malvarma aero = subject of eniris

So:

• objects often take -n
• subjects do not

That is why aero stays without -n here.

Yes. malvarma is built from:

• varma = warm
• mal- = opposite

So:

• varma = warm
• malvarma = cold

This is very common in Esperanto. The prefix mal- often creates the opposite of a word.

Examples:

• bona = good → malbona = bad
• granda = big → malgranda = small
• fermi = close → malfermi = open

Exactly.

• iri = to go
• en- = in, into
• eniri = to go in, enter

So malvarma aero eniris means cold air entered / came in.

This is a very common Esperanto pattern: a prefix changes the meaning of a basic root.

Because eniri already includes the idea of going in or entering.

So:

• aero eniris = air entered / came in

You do not need an extra en unless you want to specify the place entered:

• La aero eniris la ĉambron. = The air entered the room.
• La aero eniris en la ĉambron. = The air went into the room.

Both patterns can exist, but in your sentence no destination needs to be stated, so eniris alone is enough.

Because fumon is the direct object of sentis, and adjectives must agree with the nouns they describe.

So:

• fumo = smoke
• densan fumon = dense smoke

Both take -n because:

• fumon is accusative
• densan must match fumon in case and number

This agreement is a core Esperanto rule:

• singular noun → singular adjective
• plural noun → plural adjective
• accusative noun → accusative adjective too

Yes. senti is broader than English feel. It means to sense, feel, perceive through the senses.

So ni sentis densan fumon can mean that the people became aware of the smoke, especially by smelling it.

If you wanted to be more specific about smell, Esperanto also has:

• flari = to smell, sniff

But senti fumon is completely natural for to notice/smell smoke.

It describes the source of the smoke.

So the meaning is:

• densan fumon el la najbara apartamento = dense smoke from the neighboring apartment

Here el means from, out of.

It does not mean that we felt from the neighboring apartment. It specifically tells you where the smoke was coming from.

najbara means neighboring, adjacent, nearby.

It comes from the root najbar-, which is related to the idea of a neighbor. In this sentence:

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

So it identifies which apartment the smoke came from.

It is repeated for clarity and natural style:

• malvarma aero eniris, kaj ni sentis...

If you omitted ni, the sentence could become awkward or confusing, because the subject has changed:

• first clause subject: malvarma aero
• second clause subject: ni

Repeating ni makes it immediately clear that the second action belongs to we, not to cold air.

Yes, the word order here is quite English-like, but Esperanto is somewhat more flexible because of endings like -n.

This sentence uses a very standard, natural order:

• time clause first
• then the main events
• then the source phrase after fumon

Because -n marks objects, Esperanto can sometimes move things around more freely than English. But for learners, the order used here is the most straightforward and natural.

La is the definite article, meaning the.

It is used here because both nouns are understood as specific:

• la pordon = the door, a particular door known in the situation
• la najbara apartamento = the neighboring apartment, a specific apartment being referred to

Esperanto has only one definite article, la, and it does not change for gender, number, or case.

The commas help separate the parts of the sentence clearly.

• After pordon: this closes the opening Kiam time clause.
• After eniris: this separates two coordinated main clauses joined by kaj.

So the punctuation mirrors the structure:

1. Kiam ni malfermis la pordon,
2. malvarma aero eniris,
3. kaj ni sentis densan fumon el la najbara apartamento.

This makes the sentence easier to read and is very normal punctuation in Esperanto.