Post tiom longa tago mia humoro fariĝis pli bona nur kiam ni trinkis teon kune.

Because post is a preposition, and in Esperanto prepositions normally take the basic form without -n.

So:

• post tago = after a day
• post tiom longa tago = after such a long day

The -n ending is not used here because tago is not the direct object of a verb. It is part of a prepositional phrase.

Also, longa matches tago, so both stay without -n:

• tiom longa tago

Here tiom means something like so, that much, or to such an extent.

So tiom longa tago means:

• such a long day
• literally, something like a day that long

It emphasizes degree. The speaker is not just saying the day was long, but very or remarkably long.

Both can express strong emphasis, but they are not exactly the same.

• tre longa tago = a very long day
• tiom longa tago = such a long day / so long a day

Tre is just an intensifier: very.
Tiom points more to degree or extent: so much, to that degree, such.

In this sentence, tiom longa tago sounds natural because the speaker is stressing just how long the day felt.

Because the thing that changed is specifically the speaker's mood, not the speaker as a whole.

• mi fariĝis pli bona would mean I became better
• mia humoro fariĝis pli bona means my mood became better

Esperanto often says exactly what changed, even when English might sometimes phrase it more loosely.

Because fariĝi means to become, while esti means to be.

So:

• mia humoro estis pli bona = my mood was better
• mia humoro fariĝis pli bona = my mood became better

The sentence is about a change of state. The mood was not good before, and then it improved. That is why fariĝis is the right choice.

Because bona describes humoro, which is a noun.

• humoro = noun
• bona = adjective describing that noun

So:

• mia humoro fariĝis pli bona = my mood became better

If you used bone, that would be an adverb, and adverbs describe verbs, adjectives, or other adverbs, not nouns.

Compare:

• Li kantas bone = He sings well
• La kanto estas bona = The song is good

Here the mood itself is better, so the adjective bona is needed.

In Esperanto, pli does not always need an explicit than phrase.

Here, pli bona means better in comparison with the earlier state of the mood. The comparison is understood from context.

If you wanted to say what it is better than explicitly, you could use ol:

• pli bona ol antaŭe = better than before

But in this sentence, that extra comparison is unnecessary because it is obvious: the mood became better than it had been earlier.

Nur kiam means only when.

It tells us that the mood improved at that moment and not before.

So the sense is:

• After such a long day, my mood improved only when we drank tea together.

This creates a strong limitation:

• not during the day
• not before meeting
• not from resting alone
• only when the tea-drinking together happened

Because teon is the direct object of trinkis.

In Esperanto, the direct object takes -n:

• trinki teon = to drink tea
• ni trinkis teon = we drank tea

The verb trinki acts directly on what is drunk, so that noun gets the accusative ending.

Because kune is an adverb meaning together, while kun is a preposition meaning with.

In this sentence, the idea is that we drank tea together, so the adverb kune is the right form.

Compare:

• Ni trinkis teon kune = We drank tea together
• Mi trinkis teon kun ŝi = I drank tea with her

So:

• use kune when you mean together
• use kun when it is followed by a noun or pronoun: kun mi, kun amiko, kun ŝi

Because both actions are presented as past events.

• fariĝis = became
• trinkis = drank

The sentence is narrating something that happened in the past: after a long day, the speaker's mood improved when they drank tea together.

Esperanto often uses the simple past -is for both the main event and the event in the time clause, just as English often does:

• My mood became better only when we drank tea together

The relationship between the events is clear from the meaning, so no more complicated tense is needed.

No, Esperanto word order is fairly flexible, although some orders sound more natural than others.

This sentence is natural because it begins with the time-setting phrase:

• Post tiom longa tago = After such a long day

Then it gives the subject and main change:

• mia humoro fariĝis pli bona

Then the limiting condition:

• nur kiam ni trinkis teon kune

So the order helps the sentence flow well:

1. background
2. main statement
3. exact moment/cause of improvement

A different order could still be grammatical, but it might change the emphasis. For example, putting nur kiam earlier would highlight the restriction more strongly.

Because possessive words like mia, via, lia, and so on usually replace the article la.

So Esperanto normally says:

• mia humoro = my mood not
• la mia humoro

This works much like English: we say my mood, not the my mood.

Yes, Esperanto allows some movement of adverbs like kune, but not every position sounds equally natural.

These are possible in principle:

• ni trinkis teon kune
• ni kune trinkis teon

Both are fine.

The version in the sentence, trinkis teon kune, sounds very natural and straightforward. It places kune after the action, where English speakers also often expect together.

So the meaning stays the same, but the rhythm and emphasis can shift a little.