Kiam mia avo venas, li ĉiam rakontas amuzan rakonton.

Kiam means when, and here it introduces a time clause: Kiam mia avo venas = When my grandfather comes.

In Esperanto, it is very common to put this kind of clause first, just like in English:

• When my grandfather comes, he always tells a funny story.

You could also put the main clause first in Esperanto:

• Li ĉiam rakontas amuzan rakonton, kiam mia avo venas.

That is also grammatical, though the original version sounds very natural.

Esperanto often uses the present tense for actions that are habitual or generally true.

So Kiam mia avo venas, li ĉiam rakontas... means something like:

• whenever my grandfather comes
• when my grandfather comes over
• each time he comes

This is not mainly about one specific future visit. It describes a repeated pattern. Because of that, venas is the normal choice.

If you used venos, it would sound more like a specific future event:

• Kiam mia avo venos, li rakontos...
• When my grandfather comes / will come, he will tell...

So the present tense here is used because the sentence expresses a habit.

The comma separates the subordinate clause from the main clause.

• Kiam mia avo venas = subordinate time clause
• li ĉiam rakontas amuzan rakonton = main clause

Esperanto commonly uses a comma between these parts, especially when the subordinate clause comes first. This is very similar to English punctuation in sentences like:

• When my grandfather comes, he always tells a funny story.

Because possessive words like mia already make the noun definite enough.

• mia avo = my grandfather

In Esperanto, you normally do not add la before a possessive:

• mia libro = my book
• ŝia domo = her house
• nia instruisto = our teacher

So la mia avo would be wrong in normal Esperanto.

Esperanto normally requires an explicit subject in each clause.

So after the first clause, the second clause needs its own subject:

• Kiam mia avo venas, li ĉiam rakontas...

Here, li refers back to mia avo.

English also does this:

• When my grandfather comes, he always tells...

You usually cannot just say:

• Kiam mia avo venas, ĉiam rakontas...

because then the second clause would be missing its subject.

Ĉiam means always, and adverbs in Esperanto are fairly flexible in position, though some placements are more natural than others.

In this sentence, li ĉiam rakontas is a very natural order:

• subject + adverb + verb

You may also see:

• Li rakontas ĉiam amuzan rakonton.

But li ĉiam rakontas sounds more standard and smoother.

So this is mostly a matter of natural word order, not a special grammar rule just for ĉiam.

Because rakonton is the direct object, and the adjective modifying it must agree with it.

Breakdown:

• rakonto = story
• rakonton = story, as a direct object
• amuza = funny/amusing
• amuzan = funny/amusing, agreeing with a direct object noun

In Esperanto, adjectives agree with nouns in both:

• number
• case

So if the noun takes -n, the adjective takes -n too:

• amuza rakonto = a funny story
• amuzan rakonton = a funny story, as the object

The -n marks the accusative, which usually shows the direct object of the verb.

Here the verb is rakontas = tells.
What is being told? amuzan rakonton.

So:

• li rakontas rakonton = he tells a story

The word receiving the action of the verb gets -n.

This lets Esperanto keep word roles clear even if the word order changes.

For example, these mean the same thing:

• Li rakontas amuzan rakonton.
• Amuzan rakonton li rakontas.

The -n shows that rakonton is still the object.

Because amuza rakonto would be the basic dictionary-style form, but in the sentence the phrase is the direct object.

Compare:

• Tio estas amuza rakonto. = That is a funny story.
  Here it is not a direct object, so no -n.

• Li rakontas amuzan rakonton. = He tells a funny story.
  Here it is the direct object, so both words get -n where appropriate.

So the difference is not about meaning, but about grammatical role in the sentence.

It may feel a little repetitive to an English speaker because tell a story is idiomatic in English, while Esperanto uses the root rakont- for both:

• rakonti = to tell, narrate
• rakonto = story, tale

So rakontas rakonton is perfectly normal Esperanto. It is basically tells a story.

Esperanto often allows this kind of verb + related noun combination naturally. It is not considered bad style here.

If you wanted, you could rephrase it in other ways, but the original sentence is completely ordinary and correct.

Esperanto has no indefinite article. English has a/an, but Esperanto simply leaves it out.

So:

• rakonto can mean a story
• amuzan rakonton can mean a funny story

Esperanto does have a definite article:

• la = the

So if you wanted the funny story, you would say:

• la amuzan rakonton only if it is a direct object phrase, though more basically:
• la amuza rakonto = the funny story
• li rakontas la amuzan rakonton = he tells the funny story

In the original sentence, no article is needed because the meaning is indefinite.

Yes, to some extent. Esperanto word order is more flexible than English because endings show grammatical roles.

The neutral order here is:

• Kiam mia avo venas, li ĉiam rakontas amuzan rakonton.

But some variations are possible:

• Li ĉiam rakontas amuzan rakonton, kiam mia avo venas.
• Amuzan rakonton li ĉiam rakontas, kiam mia avo venas.

These still work because:

• li is clearly the subject
• rakonton is clearly the object because of -n

That said, the original version is the most natural straightforward phrasing for a learner to imitate.

Here is a quick breakdown:

• Kiam — conjunction/adverb meaning when
• mia — possessive pronoun/adjective meaning my
• avo — noun meaning grandfather
• venas — verb, present tense, comes
• li — pronoun, he
• ĉiam — adverb, always
• rakontas — verb, present tense, tells
• amuzan — adjective, accusative singular, funny/amusing
• rakonton — noun, accusative singular, story

This sentence is a good example of several core Esperanto patterns at once:

• present tense -as
• adjective ending -a
• noun ending -o
• adverb ending -e in general, though ĉiam is a special adverb form
• accusative -n
• adjective agreement with the noun it describes