La koltuko kaj la ŝtrumpoj, kiujn mia onklino aĉetis, estas tiel varmaj, ke mi ne plu plendas pri la vintro.

In Esperanto, la is the definite article, meaning the. It does not change for gender, number, or case.

Here, la koltuko kaj la ŝtrumpoj means the scarf and the socks.

It is very common to repeat la before each noun in a pair:

• la koltuko kaj la ŝtrumpoj

You could sometimes omit the second la if the structure is very clear, but repeating it is normal and often sounds better.

Esperanto marks the plural with -j.

So:

• ŝtrumpo = sock
• ŝtrumpoj = socks

That final -j is the regular plural ending for nouns.

Actually, estas works for both singular and plural subjects. Esperanto verbs do not change according to number or person.

So:

• la koltuko estas varma = the scarf is warm
• la ŝtrumpoj estas varmaj = the socks are warm
• la koltuko kaj la ŝtrumpoj estas varmaj = the scarf and the socks are warm

The verb stays estas in every case.

Adjectives in Esperanto must agree with the nouns they describe.

Since la koltuko kaj la ŝtrumpoj is a plural idea overall, the adjective also takes the plural ending -j:

• varma = warm
• varmaj = warm (plural)

So estas tiel varmaj means are so warm.

Kiujn is a relative pronoun meaning which or that, referring back to la koltuko kaj la ŝtrumpoj.

It has:

• kiuj- because it refers to something plural
• -n because it is the direct object inside the relative clause

The relative clause is:

• kiujn mia onklino aĉetis = which my aunt bought

In that clause:

• mia onklino = subject
• aĉetis = bought
• kiujn = object

So the -n shows that the aunt bought them.

In Esperanto, ke means that when introducing a content clause, like:

• Mi scias, ke li venos. = I know that he will come.

But here we need a relative word referring back to nouns:

• the scarf and the socks, which/that my aunt bought

For that, Esperanto uses a relative pronoun such as kiu, kiuj, kiun, kiujn, depending on number and case.

So:

• ke introduces an idea or statement
• kiujn refers back to specific things

Yes. It refers to both nouns together:

• la koltuko kaj la ŝtrumpoj

Because the reference is plural, Esperanto uses kiujn rather than singular kiun.

So the meaning is:

• the scarf and the socks, which my aunt bought

Esperanto word order is fairly flexible because endings show grammatical roles.

In this clause:

• kiujn has -n, so it is the object
• mia onklino has no -n, so it is the subject

Therefore:

• kiujn mia onklino aĉetis
• mia onklino aĉetis kiujn

would both be grammatically understandable, though the first is the normal order in a relative clause.

English depends more on word order. Esperanto depends more on endings.

Tiel ... ke is a very common Esperanto pattern meaning so ... that.

In this sentence:

• estas tiel varmaj, ke mi ne plu plendas
  = are so warm that I no longer complain

Examples:

• Li estas tiel laca, ke li tuj endormiĝis. = He is so tired that he fell asleep immediately.
• La akvo estas tiel malvarma, ke mi ne volas naĝi. = The water is so cold that I do not want to swim.

So:

• tiel = so
• ke = that

Good question. They are related, but not the same.

• tre varmaj = very warm
• tiel varmaj, ke ... = so warm that ...

Tre simply intensifies an adjective:

• La ŝtrumpoj estas tre varmaj. = The socks are very warm.

Tiel ... ke introduces a result:

• La ŝtrumpoj estas tiel varmaj, ke mi ne plendas. = The socks are so warm that I do not complain.

So in this sentence, tiel is used because the speaker is giving a consequence.

Ne plu means no longer or not anymore.

So:

• mi ne plu plendas = I no longer complain

The word plu has the idea of further / any longer / anymore, and with ne it gives the meaning not anymore.

Examples:

• Mi ne plu loĝas tie. = I no longer live there.
• Ŝi ne plu laboras ĉi tie. = She no longer works here.

Pri usually means about or concerning.

So:

• plendi pri io = to complain about something

In this sentence:

• mi ne plu plendas pri la vintro = I no longer complain about the winter

This is a fixed and common combination:

• paroli pri = talk about
• pensi pri = think about
• plendi pri = complain about

Because la vintro is not the direct object of the verb. It is the object of the preposition pri.

In Esperanto, nouns after prepositions normally do not take -n unless there is some special reason, especially motion in certain expressions.

So:

• plendi pri la vintro = complain about the winter

No accusative is needed there.

Aĉetis is past tense.

Esperanto verb endings are very regular:

• -as = present
• -is = past
• -os = future
• -us = conditional
• -u = imperative / volitive
• -i = infinitive

So:

• aĉeti = to buy
• aĉetas = buys / is buying
• aĉetis = bought

Here:

• mia onklino aĉetis = my aunt bought

Because mia onklino is the subject of aĉetis in the relative clause.

In:

• kiujn mia onklino aĉetis

the roles are:

• kiujn = object, because of -n
• mia onklino = subject

Only the direct object gets -n here.

In standard Esperanto punctuation, a relative clause is often separated by commas, especially when it is inserted into the main clause.

So:

• La koltuko kaj la ŝtrumpoj, kiujn mia onklino aĉetis, estas ...

This is normal and clear.

Punctuation can vary somewhat in Esperanto, but this comma usage is very common and natural.

They could be in a different order if the meaning remains the same:

• La ŝtrumpoj kaj la koltuko ...

There is no grammar rule forcing this order. The chosen order may simply reflect what the speaker wants to mention first, what feels natural, or what is more important in context.

So the order here is stylistic, not grammatical.