Ĉiu sezono havas ion belan, sed en somero ni plej ofte sidas ĉe la rivero.

Ĉiu means each/every, but it treats the noun as singular: each season.

So:

• ĉiu sezono = each season / every season
• ĉiuj sezonoj = all the seasons

Both can be correct in other contexts, but they are slightly different in perspective:

• Ĉiu sezono havas ion belan = Every single season has something beautiful.
• Ĉiuj sezonoj havas ion belan = All the seasons have something beautiful.

Esperanto often uses singular after ĉiu, just like English uses singular after each.

Because it is the direct object of havas.

In Esperanto, the thing that is directly affected by the verb usually takes -n. Here, the verb is havi (to have), and the thing being had is io bela (something beautiful), so it becomes:

• io → ion
• bela → belan

The adjective also matches the noun-like word it describes, so if io takes -n, bela takes -n too.

So:

• io bela = something beautiful
• ion belan = something beautiful, as a direct object

Because after havas, Esperanto normally marks the direct object with -n.

Compare:

• Io bela estas ĉi tie. = Something beautiful is here.
  
  • Here io bela is the subject, so no -n.
• Ĉiu sezono havas ion belan. = Every season has something beautiful.
  
  • Here ion belan is the object, so it takes -n.

So the difference is grammatical role, not meaning.

When talking about a season in a general sense, Esperanto often leaves out the article.

So:

• en somero = in summer / in the summertime / during summer
• en la somero = in the summer, usually a more specific summer or a more definite context

In this sentence, the meaning is general: during summer, we most often sit by the river. So en somero sounds natural.

You can think of it as similar to English in summer rather than in the summer, though English allows both more freely.

Yes. Somere would also be natural.

• en somero = in summer
• somere = in summertime / during summer

Both are good Esperanto. The version with en somero uses a prepositional phrase, while somere uses an adverb formed from the noun somero.

Similarly:

• printempe = in spring
• aŭtune = in autumn
• vintre = in winter

So a similar sentence could be:

• Sed somere ni plej ofte sidas ĉe la rivero.

Ofte means often, and plej makes it a superlative:

• ofte = often
• pli ofte = more often
• plej ofte = most often

So ni plej ofte sidas ĉe la rivero means we most often sit by the river or we usually sit by the river more than anywhere else / more often than at other times.

This is a very common Esperanto pattern:

• bela = beautiful
• pli bela = more beautiful
• plej bela = most beautiful

And with adverbs:

• rapide = quickly
• pli rapide = more quickly
• plej rapide = most quickly

Because it functions as an adverbial phrase modifying the verb.

In Esperanto, adverbs are fairly flexible in position, but they are often placed near the verb they modify. So:

• ni plej ofte sidas ĉe la rivero
• ni sidas plej ofte ĉe la rivero

Both are understandable, but the first version sounds smoother and more standard here.

Putting plej ofte before sidas makes it clear that it describes how often we sit.

Both can refer to being near the river, but they are not exactly the same.

• ĉe = at, by, in the vicinity of, at the place of
• apud = beside, next to

So:

• ĉe la rivero suggests at the river / by the river, a natural location phrase
• apud la rivero suggests being more specifically next to the river

In many contexts both would work, but ĉe la rivero is very idiomatic for a place where people go and spend time.

Because la marks it as definite: the river.

This usually means the speaker and listener can identify which river is meant. It may be:

• the local river near them
• the river already known from context
• the river that is relevant in their area

Without la, ĉe rivero would sound odd here, because the sentence is not talking about just any river in a vague sense. It is talking about a particular familiar place.

Sidas is the normal present tense of sidi, meaning to be sitting / to sit.

• sidi = to sit, to be seated
• sidas = sit / are sitting

Other forms would change the meaning:

• eksidas = begin to sit down
• sidantas is not standard Esperanto; the normal verb is simply sidas

Esperanto often uses simple verb forms where English might choose between sit and be sitting depending on context.

So ni sidas ĉe la rivero can mean:

• we sit by the river
• we are sitting by the river

Here, because of plej ofte, the habitual sense we usually sit by the river fits best.

Yes, in this sentence sed means but.

It links two contrasting ideas:

• Ĉiu sezono havas ion belan = Every season has something beautiful
• sed en somero ni plej ofte sidas ĉe la rivero = but in summer we most often sit by the river

So the sentence says that all seasons have something nice, but summer is the time when this particular activity happens most often.

Ĉiu is pronounced approximately CHEE-oo.

A few details:

• ĉ sounds like ch in church
• i sounds like ee
• u sounds like oo

So ĉiu is two syllables: ĉi-u.

This is useful because beginners sometimes want to read it like one English-style vowel cluster, but Esperanto pronunciation is very regular.