La ĝentileco de la kelnerino estis klara eĉ al mia patro, kiu kutime ne rimarkas tiajn detalojn.

Ĝentileco is a noun meaning politeness or courtesy.

It is built from:

• ĝentila = polite
• -ec- = the quality or abstract trait
• -o = noun ending

So:

• ĝentila = polite
• ĝentileco = politeness

The sentence needs a noun because the subject is the politeness of the waitress, not the waitress was polite.

De often means of. Here it shows whose politeness is being talked about:

• la ĝentileco de la kelnerino = the politeness of the waitress

This is a very normal Esperanto way to express possession or association. English often prefers the waitress’s politeness, but Esperanto commonly uses de.

Kelnerino means female waiter / waitress / female server.

It is built from:

• kelner- = waiter/server
• -in- = female
• -o = noun

So kelnerino specifically marks that the server is female.

Because klara describes the subject ĝentileco.

After esti (to be), Esperanto normally uses an adjective, not an adverb, when you are describing what something is:

• La ĝentileco estis klara = The politeness was clear

Here:

• ĝentileco is singular
• so the adjective is singular too: klara

If you used klare, that would be an adverb, meaning clearly, which would not fit the structure here.

Because klara is not the direct object. It is a predicate adjective linked to the subject by estis.

In Esperanto, the -n ending is usually for:

• direct objects
• motion toward something in certain expressions

Here:

• la ĝentileco is the subject
• klara describes that subject

So it stays klara, not klaran.

Because klara al means clear to someone.

So:

• klara al mia patro = clear to my father
• eĉ al mia patro = even to my father

This works much like English clear to someone.

Also note:

• mia patro does not take -n because it is the object of the preposition al, not a direct object.

In Esperanto, possessives like mia, via, lia, ŝia, and so on already make the noun definite enough in most cases.

So:

• mia patro = my father

You normally do not say la mia patro in a sentence like this.

Compare:

• la kelnerino = the waitress
• mia patro = my father

Kiu refers to mia patro.

The relative clause is:

• kiu kutime ne rimarkas tiajn detalojn

So the meaning is:

• my father, who usually does not notice such details

It does not refer to la kelnerino or ĝentileco. The comma helps show that this is extra information about my father.

Because the relative clause describes a general habit of the father:

• kiu kutime ne rimarkas tiajn detalojn = who usually does not notice such details

So the main event is in the past:

• estis = was

But the description of the father is timeless or habitual:

• rimarkas = notices / tends to notice

This is very natural in Esperanto, just as in English:

• It was clear even to my father, who usually doesn’t notice such details.

Kutime is an adverb meaning usually.

It modifies rimarkas:

• kutime ne rimarkas = usually does not notice

It tells you that this is the father’s normal habit.

Because detalojn is a plural direct object, and tiajn agrees with it.

Breakdown:

• detaloj = details
• detalojn = details as a direct object
• tiaj detaloj = such details
• tiajn detalojn = such details as a direct object

So:

• rimarki ion = to notice something
• rimarkas tiajn detalojn = notices such details

The adjective tiajn must match the noun detalojn in both:

• plural: -j
• accusative: -n

Esperanto word order is fairly flexible, but this version is the most straightforward and natural.

Current order:

• La ĝentileco de la kelnerino estis klara eĉ al mia patro...

This presents:

1. the subject
2. the verb
3. the rest of the information

You could move parts for emphasis, for example:

• Eĉ al mia patro la ĝentileco de la kelnerino estis klara

That would emphasize even to my father more strongly. But the original sentence is the clearest neutral order.