Mi reklamaciis ĉe klientservo.

-is is the Esperanto ending for the past tense.

So:

• reklamacii = to make a complaint / to lodge a complaint
• reklamaciis = made a complaint / complained

Esperanto verbs do not change for person, only for tense and mood, so -is always means past, no matter who the subject is.

It comes from the root reklamaci-, which is related to reklamacio = a complaint, especially a customer complaint or claim.

Esperanto often lets you switch between word classes very easily:

• reklamacio = complaint
• reklamacii = to complain / to file a complaint
• reklamaciis = complained / filed a complaint

So this is a very regular Esperanto pattern.

No. This is an important difference.

• reklami usually means to advertise or to promote
• reklamacii means to make a complaint or to lodge a claim

Even though the words look similar, they are not the same in meaning. A learner might easily confuse them, so it is worth remembering that reklamacio is about a complaint, not an advertisement.

Because Esperanto verbs do not show the person.

In English, complained can still work with an understood subject in some situations, but in Esperanto reklamaciis by itself does not tell you whether the subject is I, you, he, she, they, etc.

So Mi reklamaciis clearly means I complained.

Here ĉe means something like at, with, or in dealings with, depending on how you naturally translate it into English.

In Esperanto, ĉe is often used for being in contact with a person, office, business, or institution. So ĉe klientservo means that the complaint was made with / to customer service.

It does not have to mean only physical location. It can also describe the place or department where the interaction happened.

Ĉe focuses on the person or department you dealt with: with / at customer service.

Al would focus more on direction: to customer service.

In many real situations, both ideas are close, but ĉe is very natural when talking about contacting or dealing with a service desk, office, or institution. It presents customer service as the place or department where the complaint was handled.

Because klientservo is the object of the preposition ĉe.

Normally, after a preposition like ĉe, the noun does not take -n:

• ĉe klientservo
• ĉe la vendejo
• ĉe mia amiko

The -n ending is mainly used for:

1. a direct object
2. direction after certain prepositions or in place expressions

Here, klientservo is not a direct object, so no -n is needed.

Because Esperanto very often forms compound words as a single unit.

• kliento = customer/client
• servo = service
• klientservo = customer service

The final part is usually the main idea, and the earlier part modifies it. So this literally means something like customer-service.

This is extremely common in Esperanto and is one of the language’s most useful features.

Esperanto uses la only when the speaker wants to mark something as definite and identifiable.

In a sentence like this, klientservo can be understood in a more general or institutional sense, like customer service as a department or function. That is why ĉe klientservo can sound natural.

If you want to stress a particular, identifiable customer service department, ĉe la klientservo is also possible.

So:

• ĉe klientservo = at/with customer service, as a service function or department
• ĉe la klientservo = at/with the customer service department, more explicitly definite

Yes, but the meaning would be a little different.

• plendi = to complain in general
• reklamacii = to make a complaint more formally, especially as a customer about a product, service, bill, defect, or similar issue

So reklamacii sounds more specific to a customer-service context. It suggests an actual complaint or claim, not just expressing annoyance.

That makes it a very suitable word in this sentence.