Ni dankos la kelnerinon pro la bona servo.

Because -os is the Esperanto future-tense ending.

• dankas = thanks / is thanking
• dankos = will thank

So Ni dankos... means We will thank...

The -n marks the direct object.

In this sentence, the waitress is the person receiving the action of thanking, so she is the direct object:

• Ni = we
• dankos = will thank
• la kelnerinon = the waitress

Esperanto often uses -n where English relies mostly on word order.

Because -in- is the feminine suffix.

• kelnero = waiter
• kelnerino = waitress

Then the extra -n is added because it is the direct object:

• kelnerino
  • -n = kelnerinon

So the word breaks down like this:

• kelner- = waiter/server
• -in- = female
• -o = noun
• -n = direct object

La is the Esperanto definite article, meaning the.

So:

• la kelnerinon = the waitress

It shows that a specific waitress is meant, not just any waitress.

Pro means because of, on account of, or for in the sense of for the reason of.

So:

• pro la bona servo = because of the good service

This is different from por, which usually means for in the sense of for the purpose of.

For example:

• Mi laboras por vi. = I work for you.
• Mi dankas vin pro via helpo. = I thank you for your help.

In this sentence, pro gives the reason for thanking.

Because la bona servo is part of a prepositional phrase introduced by pro.

Normally, nouns after a preposition do not take the accusative -n unless there is some special reason involving direction or movement. Here there is no such reason, so we get:

• pro la bona servo = for the good service

Also, adjectives agree with their nouns, so:

• bona matches servo
• both are singular
• neither has -n

If the noun had -n, the adjective would also have -n.

Because bona is an adjective, and adjectives usually come before the noun they describe in Esperanto, just as in English.

• bona servo = good service

Esperanto word order is somewhat flexible, so servo bona is possible in some contexts, but bona servo is the normal and most natural order.

In Esperanto, danki usually takes the person as a direct object.

So:

• Mi dankas vin. = I thank you.

If you want to add the reason, you commonly use pro:

• Mi dankas vin pro la helpo. = I thank you for the help.

That is exactly the pattern used here:

• Ni dankos la kelnerinon pro la bona servo.

So the structure is:

subject + danki + person thanked + pro + reason

Yes. That is a very close, natural translation.

Word by word:

• Ni = we
• dankos = will thank
• la kelnerinon = the waitress
• pro = for / because of
• la bona servo = the good service

So the whole sentence means:

We will thank the waitress for the good service.

Yes, to some extent. Esperanto allows more flexibility than English because the -n ending marks the direct object.

For example, these would still be understandable:

• La kelnerinon ni dankos pro la bona servo.
• Pro la bona servo ni dankos la kelnerinon.

However, the original order is the most straightforward and neutral:

• Ni dankos la kelnerinon pro la bona servo.

So beginners should usually stick to this normal order unless they have a reason to emphasize something.

Here it means service in the restaurant/customer-service sense.

So la bona servo means the good service that the waitress gave.

The noun servo can refer generally to service, and the exact meaning depends on context. In this sentence, because of kelnerino, the restaurant meaning is the natural one.