Kiam eksonis la sonorilo, mi malfermis la pordon.

Here kiam means when and introduces a time clause:

Kiam eksonis la sonorilo = When the bell rang

In Esperanto, kiam can be used both:

• in questions: Kiam vi venos? = When will you come?
• in subordinate clauses: Kiam li venis, mi foriris. = When he came, I left.

So in this sentence, it is not asking a question; it is linking two actions in time.

The prefix ek- usually means beginning suddenly or starting.

So:

• soni = to sound
• sonis = sounded / was sounding
• eksonis = started to sound, rang out, went off

With a bell, eksonis is very natural because a bell often begins ringing at a specific moment. It gives the sense of the bell started ringing rather than just the bell was making sound.

Both verbs end in -is, which is the Esperanto past-tense ending:

• eksonis = rang / started ringing
• malfermis = opened

This is one of the nice regular features of Esperanto:

• -as = present
• -is = past
• -os = future

And this does not change for different persons. So mi malfermis, li malfermis, and ili malfermis all use the same past-tense ending.

Because la pordon is the direct object, while la sonorilo is the subject.

• la sonorilo = the bell → the thing doing the ringing
• la pordon = the door → the thing being opened

In Esperanto, the direct object usually gets -n:

• Mi malfermis la pordon. = I opened the door.
• Mi vidis la hundon. = I saw the dog.

The subject does not get -n:

• La sonorilo eksonis.

So the -n on pordon tells you that the door is what was opened.

malfermis comes from:

• ferm- = close
• mal- = opposite
• -i = infinitive verb ending

So:

• fermi = to close
• malfermi = to open

Then the past ending -is gives:

• malfermis = opened

This is a very common Esperanto pattern. Instead of learning completely unrelated opposite words, you often add mal-:

• bona = good → malbona = bad
• varma = warm → malvarma = cold
• fermi = close → malfermi = open

sonorilo is built from:

• sonor- = sound, ring
• -il- = instrument or tool
• -o = noun

So sonorilo literally means something like a sounding instrument or a ringing device. In normal usage, it means bell, and depending on context it can also refer to something like a doorbell.

This kind of word-building is very common in Esperanto:

• tranĉi = to cut → tranĉilo = knife
• skribi = to write → skribilo = writing instrument

La is the Esperanto definite article, meaning the.

So:

• la sonorilo = the bell
• la pordo = the door

Esperanto has only one article, la. It does not have a separate word for a/an.

Whether English uses the or leaves it more implicit can depend on context, but in Esperanto la is used when the speaker has a specific thing in mind. In this sentence, it is clearly a particular bell and a particular door.

Both are possible.

• Kiam eksonis la sonorilo, mi malfermis la pordon.
• Kiam la sonorilo eksonis, mi malfermis la pordon.

Esperanto word order is fairly flexible. Putting the verb first here can sound a little more narrative or dramatic, as if the event begins with the ringing itself.

A learner should probably be most comfortable first with the more familiar order:

• Kiam la sonorilo eksonis...

But the version with the verb first is completely normal.

Yes, it is standard and helpful.

The first part, Kiam eksonis la sonorilo, is a subordinate clause. The second part, mi malfermis la pordon, is the main clause. A comma is commonly used to separate them, especially when the subordinate clause comes first.

So this punctuation makes the structure easier to read:

• When the bell rang, I opened the door.

More specifically, the Esperanto suggests when it started ringing, because of ek-.

So the nuance is a little closer to:

• When the bell started to ring, I opened the door than to
• While the bell was ringing, I opened the door

However, in natural translation, English often just says When the bell rang, I opened the door, because that sounds normal and usually matches the meaning well.

If Esperanto wanted to make as soon as especially clear, it could use tuj kiam:

• Tuj kiam eksonis la sonorilo, mi malfermis la pordon.

You can analyze it that way to understand the grammar, but not to produce natural English.

A more literal breakdown is:

• Kiam = when
• eksonis = started sounding / rang out
• la sonorilo = the bell
• mi malfermis la pordon = I opened the door

So the sentence structure makes sense word by word, but a natural English translation is simply:

• When the bell rang, I opened the door.

That is often the best approach with Esperanto: understand the pieces clearly, but translate into normal English rather than overly literal English.