La baldaŭ komenciĝonta leciono estos pri tio, kiel oni devas iri, kiam sur la stratoj estas glacio.

Komenciĝonta is a participial adjective.

It is built like this:

• komenc- = start, begin
• -iĝ- = become, get into a state; here it makes the verb intransitive, so komenciĝi = to begin
• -ont- = future active participle, meaning about to do or going to do
• -a = adjective ending

So komenciĝonta means about to begin or soon going to begin.

Because it ends in -a, it works like an adjective and modifies leciono.

Because -ant- means doing something now, while -ont- means going to do something later.

So:

• komencanta leciono = a lesson that is beginning right now / already in the process of beginning
• komenciĝonta leciono = a lesson that is about to begin

In this sentence, the lesson has not started yet, so -ont- is the right choice.

Because komenci is normally transitive: someone starts something.

• Mi komencas la lecionon = I start the lesson

But here the lesson itself is the thing that begins, so Esperanto uses the intransitive form:

• La leciono komenciĝas = The lesson begins

That is why the participle is based on komenciĝi, not komenci.

Komencota would be a future passive participle, meaning about to be started.

That is slightly different from komenciĝonta:

• komenciĝonta leciono = a lesson that is about to begin
• komencota leciono = a lesson that is about to be started by someone

In many situations the meanings are close, but komenciĝonta focuses on the lesson entering the state of beginning, while komencota highlights an outside agent who will start it.

Because baldaŭ is an adverb, and it modifies the idea of beginning.

So baldaŭ komenciĝonta means about to begin soon or more naturally soon-to-begin.

Esperanto often allows an adverb to modify a participle like this. A longer equivalent would be:

• la leciono, kiu baldaŭ komenciĝos

The participial version is just more compact.

In Esperanto, adjectives usually come before the noun they modify, and participles used as adjectives behave the same way.

So:

• la baldaŭ komenciĝonta leciono

is the normal, compact order.

You can sometimes put adjectives after the noun, but that is less neutral and often more stylistic:

• la leciono baldaŭ komenciĝonta

A learner will most often see the pre-noun order first.

Because the sentence is referring to a specific lesson, not just any lesson.

La marks definiteness, like English the.

So this is not just a lesson that will begin soon, but the lesson that is about to begin soon.

Esperanto has:

• la = the
• no separate word for a/an

So an indefinite version would simply be:

• baldaŭ komenciĝonta leciono

Esti pri means to be about, to concern, or to deal with.

So:

• La leciono estos pri tio... = The lesson will be about that...

The word tio is there because pri is a preposition, and prepositions normally take a noun or pronoun object. Then the clause after it explains what tio refers to:

• pri tio, kiel oni devas iri...

So literally it is something like:

• about that, namely how one must walk...

This is a very common Esperanto pattern:

• Mi ĝojas pri tio, ke vi venis = I’m glad that you came
• Ni parolis pri tio, kion fari = We talked about what to do

Because the lesson is about the manner or way in which one should go or walk.

Kiel means how in the sense of manner.

So:

• kiel oni devas iri = how one must go / how one should walk

If you used ke, it would just introduce a statement, not a question-like clause of manner. Here the idea is not simply that one must walk, but how one must walk.

Oni is an impersonal pronoun. It means things like:

• one
• people
• you in a general sense
• anyone

So oni devas iri means something like:

• one must walk
• people must walk
• you should walk, in general

It does not refer to one specific person. It is used for general rules, advice, and statements about what people in general do.

Also, oni always takes singular verb forms:

• oni devas
• oni iras

not plural forms.

After a modal verb like devas, Esperanto uses the infinitive.

So:

• devas iri = must go / must walk

This is exactly parallel to forms like:

• povas veni = can come
• volas manĝi = wants to eat

A form like iru would be a direct command:

• Iru! = Go! / Walk!

But this sentence is not directly commanding someone. It is talking about the topic of the lesson: the proper way one is supposed to walk under certain conditions.

Kiam means when, and here it refers to the situation or time in which the advice applies:

• kiam sur la stratoj estas glacio = when there is ice on the streets

That works well for general situations such as winter conditions or recurring circumstances.

If you used se, the meaning would be closer to if:

• se sur la stratoj estas glacio = if there is ice on the streets

That is also possible in some contexts, but kiam sounds natural when talking about the circumstances in which the advice is relevant.

Yes. Esperanto does not need a dummy subject like English there.

English says:

• There is ice on the streets

Esperanto simply says:

• Sur la stratoj estas glacio
• or Estas glacio sur la stratoj

Both are normal.

Putting sur la stratoj first gives the location first, almost like setting the scene: On the streets, there is ice.

So the structure is completely natural in Esperanto.

Because the ice is physically on the surface of the streets.

• sur = on
• en = in

So:

• sur la stratoj estas glacio = there is ice on the streets

Using en would suggest something more like being inside the streets, which is not the intended image.

Because glacio here is being used as a mass noun in a general sense: ice as a substance.

So Esperanto naturally says:

• estas glacio = there is ice

not:

• estas la glacio

You would use la glacio only if you meant some specific, already identified ice, for example:

• La glacio sur la ŝtuparo estas tre danĝera = The ice on the staircase is very dangerous

In this sentence, it just means that ice is present on the streets, so no article is needed.

Because Esperanto normally uses commas to mark subordinate clauses quite clearly.

So here we get:

• pri tio, kiel oni devas iri
• iri, kiam sur la stratoj estas glacio

This is more regular than English, where comma use before subordinate clauses is often less consistent.

The commas help show the structure:

1. the lesson will be about something
2. that something is how one must walk
3. and that walking is considered when there is ice on the streets