Post kiam ni revenos el la arbaro, ni trinkos teon hejme.

It may look strange from English, but post kiam is the normal Esperanto way to say after before a whole clause.

• post by itself is a preposition, so it normally goes before a noun or noun phrase:
  • post la manĝo = after the meal
  • post nia reveno = after our return
• If you want after
  • a full clause, Esperanto uses post kiam:
    • Post kiam ni revenos... = After we return...

So post kiam is not awkward Esperanto; it is a standard pattern.

Because the returning has not happened yet. Esperanto often uses the future tense in both parts when both actions are in the future.

• ni revenos = we will return
• ni trinkos = we will drink

In English, we usually say After we return, we will drink tea, not After we will return...
Esperanto is different here: using -os in the time clause is completely normal.

Yes, in a way. Esperanto and English handle future time clauses differently.

English:

• After we return, we will drink tea.

Esperanto:

• Post kiam ni revenos, ni trinkos teon.

A native English speaker may want to copy English and say Post kiam ni revenas..., but that changes the sense and is usually not what you want here. For a future event, revenos is the natural choice.

Reveni means to come back / to return. In many contexts, English could translate it as either come back or go back, depending on viewpoint.

So:

• ni revenos = we will return
• also naturally: we will come back

Esperanto does not force the same distinction English sometimes makes between come back and go back.

Because el means out of / from inside, which fits a forest very well.

• el la arbaro = out of the forest, from the forest

By contrast, de often means from in the sense of from a person, from a place as a point of origin, or away from something, but el is the usual choice when the idea is coming out from within something.

So revenos el la arbaro strongly suggests returning from being inside the forest.

Because teon is the direct object of trinkos.

• trinki = to drink
• What will we drink? teon

In Esperanto, direct objects take the accusative ending -n:

• teo = tea
• teon = tea as the thing being drunk

This is one of the most important uses of -n.

Because hejme is an adverb-like form meaning at home.

Compare:

• hejmo = home (a noun)
• hejme = at home
• hejmen = home, to home, homewards

So:

• Ni trinkos teon hejme. = We will drink tea at home.
• Ni iros hejmen. = We will go home.

This is a very common Esperanto pattern with place words.

Yes. Esperanto word order is fairly flexible, as long as the grammar stays clear.

For example, these are possible:

• Post kiam ni revenos el la arbaro, ni trinkos teon hejme.
• Ni trinkos teon hejme post kiam ni revenos el la arbaro.

Both mean the same thing. The version with post kiam... first emphasizes the time setting: first we hear when it will happen, then what will happen.

Because the first part is a subordinate time clause:

• Post kiam ni revenos el la arbaro = after we return from the forest

Then comes the main clause:

• ni trinkos teon hejme = we will drink tea at home

Esperanto commonly uses a comma to separate such clauses, much like English does.

Usually, yes. La arbaro means the forest, so it sounds like a known or identifiable forest in the context.

• arbaro = forest
• la arbaro = the forest

If no specific forest were meant, you might see just arbaro in some contexts, but el la arbaro is very natural when both speaker and listener know which forest is being talked about.

Yes, that is possible:

• Ni trinkos hejme teon.

But Ni trinkos teon hejme is usually the more neutral and natural order. Putting hejme earlier can give slightly more emphasis to the place.

Because Esperanto marks the direct object with -n, the sentence remains clear even if the order changes.

Ni means we, but like English we, it can be either:

• inclusive: speaker + listener
• exclusive: speaker + someone else, not necessarily the listener

Esperanto does not have separate words for these two meanings. The context tells you which is meant.

Yes. That is a very good alternative.

• Post kiam ni revenos el la arbaro... = After we return from the forest...
• Post nia reveno el la arbaro... = After our return from the forest...

The first version uses a clause. The second turns the action into a noun phrase (nia reveno = our return). Both are correct; the original sentence is simply a bit more verbal and direct.