Bonaj najbaroj helpas unu la alian, eĉ kiam ili ne konas ĉiun detalon de la problemo.

Because the sentence is making a general statement about good neighbors as a group. In Esperanto, you often leave out la when speaking about something in general.

• Bonaj najbaroj helpas... = good neighbors, generally, help...
• La bonaj najbaroj helpas... = the good neighbors, meaning some specific neighbors already known in the conversation

So the version in the sentence is the natural way to say a general truth.

The ending -j marks the plural in Esperanto.

• najbaro = neighbor
• najbaroj = neighbors

Adjectives must agree with the nouns they describe in number, so:

• bona najbaro = a good neighbor
• bonaj najbaroj = good neighbors

That is why both words have -j.

In Esperanto, verbs do not change according to the subject the way they do in English.

The ending -as always means present tense:

• mi helpas = I help
• vi helpas = you help
• li helpas = he helps
• ili helpas = they help

So helpas simply means help / helps, and the subject bonaj najbaroj tells you who is helping.

Unu la alian is the standard Esperanto expression for each other or one another.

Literally, it comes from the idea one ... the other.

So:

• ili helpas unu la alian = they help each other

It is a very common reciprocal expression in Esperanto.

In this fixed reciprocal expression, only alian takes the -n because it functions as the direct object.

• alia = other
• alian = other, as a direct object

So:

• unu la alian = each other

You should learn this as a set phrase. Esperanto speakers normally say unu la alian, not unun la alian.

Also, alian is used rather than alio because alia is an adjective-like word meaning other, and in this expression it stands in for the other person.

Because sin is reflexive, while unu la alian is reciprocal.

• sin = oneself / themselves
• unu la alian = each other

So:

• Ili helpas sin would mean they help themselves
• Ili helpas unu la alian means they help each other

That is an important difference.

Eĉ means even, and kiam means when.

Together, eĉ kiam means even when.

It introduces a situation where the main statement is still true:

• Bonaj najbaroj helpas unu la alian, eĉ kiam...
• Good neighbors help each other, even when...

So eĉ adds emphasis: the helping continues despite that situation.

A learner often wonders about koni versus scii.

A useful rule is:

• koni = to know, be familiar with
• scii = to know a fact, know that, know how

In this sentence, ne konas ĉiun detalon suggests not being familiar with every detail of the situation.

You could also hear something like ne scias ĉiujn detalojn, which would focus more on not knowing all the facts. So koni is natural here, but the choice also gives a slight nuance of familiarity rather than bare factual knowledge.

Because ĉiu + singular noun means each / every, while ĉiuj + plural noun means all.

• ĉiun detalon = every detail
• ĉiujn detalojn = all the details

Both can make sense in some contexts, but they are not exactly the same.

In this sentence, ĉiun detalon matches the idea of every detail, considering the details one by one.

Because the whole phrase ĉiun detalon is the direct object of konas.

• koni ion = to know something

So:

• detalo becomes detalon
• ĉiu also agrees and becomes ĉiun

Esperanto agreement applies not only to plural -j, but also to the accusative -n when a word modifies a noun.

Here de means something like of.

• detalo de la problemo = a detail of the problem

So de la problemo tells you what the detail belongs to or is connected with.

The la shows that it is the specific problem being talked about, not just any problem in general.

Because eĉ kiam ili ne konas ĉiun detalon de la problemo is a subordinate clause.

In Esperanto, it is normal to separate such clauses with a comma, especially when they come after the main clause.

So the structure is:

• main clause: Bonaj najbaroj helpas unu la alian
• subordinate clause: eĉ kiam ili ne konas ĉiun detalon de la problemo

The comma helps show that division clearly.