Dum la ekskurso la vetero estis jen varma, jen malvarma.

Dum means during. It introduces a time phrase: dum la ekskurso = during the excursion / during the trip.

In this sentence, it tells you when the weather kept changing.

La means the, so la ekskurso refers to a specific excursion that speaker and listener already have in mind.

• dum la ekskurso = during the excursion
• dum ekskurso would sound more general, like during an excursion or while on an excursion

So here la makes the situation definite and specific.

Esperanto uses la for definite nouns, just like English uses the. Here la vetero means the weather, meaning the weather in that particular situation.

This is very natural, just like English:

• the weather was warm
• la vetero estis varma

Estis is the past tense of esti, which means to be.

So:

• estas = is / are
• estis = was / were
• estos = will be

Here la vetero estis ... means the weather was ...

Jen ... jen ... is a very useful Esperanto pattern meaning something like:

• now ... now ...
• at one moment ... at another ...
• alternately ...
• sometimes ... sometimes ...

So jen varma, jen malvarma means the weather kept shifting: it was warm at one time, cold at another.

It does not mean both at once. It suggests alternation or repeated change.

Because the sentence is not simply listing two qualities together. It is saying the weather alternated between them.

• varma kaj malvarma would sound like warm and cold, which is odd if you mean changing back and forth
• jen varma, jen malvarma clearly expresses that alternation

So jen ... jen ... is the right structure here.

Because they are adjectives.

In Esperanto:

• nouns end in -o
• adjectives end in -a
• adverbs end in -e

Here varma and malvarma describe la vetero, so they are adjectives.

Also, after a linking verb like esti, Esperanto still uses the adjective form:

• la vetero estas varma
• la vetero estis malvarma

Malvarma is built from:

• varma = warm
• mal- = opposite

So:

• varma = warm
• malvarma = cold

This is a very common Esperanto word-building pattern. The prefix mal- creates the opposite meaning:

• bona = good → malbona = bad
• granda = big → malgranda = small
• juna = young → maljuna = old

Because Esperanto often avoids repeating words when the meaning is clear.

So instead of saying:

• la vetero estis jen varma, jen estis malvarma

you simply say:

• la vetero estis jen varma, jen malvarma

Both adjectives belong to the same verb estis. This is natural and efficient.

No, Esperanto word order is fairly flexible. This version starts with the time phrase dum la ekskurso, which sets the scene first.

You could also say things like:

• La vetero dum la ekskurso estis jen varma, jen malvarma.
• Jen varma, jen malvarma estis la vetero dum la ekskurso.

But Dum la ekskurso la vetero estis jen varma, jen malvarma is a very natural, clear order.

Ekskurso usually means an excursion, outing, or trip, often a planned one, often for pleasure, school, nature, or sightseeing.

Depending on context, English might translate it as:

• excursion
• outing
• trip
• hike

So the exact English word can vary, but in Esperanto ekskurso is a normal general word for that kind of outing.