Learn Hardcore Esperanto: Hejme mi metas la bileton en la libron, por ke mi ne forgesu la daton de la koncerto. - At home I put the ticket in the book so that I do not forget the date of the concert.

You could say en la hejmo (“in the home”) or ĉe la hejmo (“at the home”), but for ordinary “at home”, hejme is the normal, idiomatic form and sounds more natural and less heavy.

Why does bileton end in -n?

Because la bileton is the direct object of the verb metas (puts).

Basic noun ending: bileto (ticket)
Add -n for the accusative (direct object): bileton

The pattern is:

Mi havas bileton. – I have a ticket.
Mi metas la bileton. – I put the ticket.

So here, la bileton is what is being put, so it takes -n.

Why does la libron also have -n after en, instead of just en la libro?

With some prepositions, especially en, Esperanto uses -n to show movement into a place, not just being located there.

en la libro = in the book (location, no movement)
en la libron = into the book (movement/direction)

In mi metas la bileton en la libron:

The ticket starts outside the book.
You move it into the book.
So en la libron uses the accusative of direction.

Both en la libro and en la libron can be heard, but for a clear idea of into, en la libron is the standard and clearer choice here, matching the idea of putting something into the book.

Could the word order be different, like Mi hejme metas la bileton en la libron?

Yes. Esperanto word order is fairly flexible. The original sentence:

Hejme mi metas la bileton en la libron…

puts hejme first for emphasis: At home (and not somewhere else) I put the ticket…

Other possible orders:

Mi hejme metas la bileton en la libron…
Mi metas hejme la bileton en la libron… (less usual, but understandable)
Mi metas la bileton en la libron hejme… (would sound a bit odd, because “in the book at home” is a strange grouping)

The safest and most natural for this style is to keep Hejme at the beginning or directly after mi. Starting with Hejme nicely sets the scene.

What does por ke mean here?

Por ke introduces a purpose clause: “so that / in order that”.

In this sentence:

…por ke mi ne forgesu la daton de la koncerto.
= “…so that I don’t forget the date of the concert.”

So the pattern is:

[main action] por ke [result you want / goal].
e.g. Mi lernas Esperanton, por ke mi povu paroli kun homoj el multaj landoj.
– I learn Esperanto so that I can speak with people from many countries.

Why is it forgesu and not forgesas or forgesi?

Forgesu is the -u form, often called the volitive or subjunctive in Esperanto. After por ke, when you express a desired or intended result, you normally use -u:

por ke mi ne forgesu = so that I (will) not forget / in order that I not forget

Comparisons:

forgesas – simple present: I forget (statement of fact)
forgesi – infinitive: to forget
forgesu – volitive: I (should / may / not) forget (wish, command, purpose, etc.)

With por ke, forgesas would be ungrammatical, and forgesi doesn’t fit because por ke needs a finite verb (a full clause), not just an infinitive.

Is forgesu here an imperative, like telling myself “Don’t forget!”?

The form is the same as the imperative, but the function is different.

As an imperative: Ne forgesu la daton! – Don’t forget the date! (a command)
In our sentence: …por ke mi ne forgesu la daton… – so that I don’t forget the date (a purpose, not a command)

Esperanto uses the -u form (volitive) for several ideas: commands, wishes, and goals/purposes. Here it’s the goal/result of putting the ticket in the book, not self-commanding.

Could I say por ne forgesi la daton instead of por ke mi ne forgesu la daton?

Yes, that’s also correct and natural:

Hejme mi metas la bileton en la libron, por ne forgesi la daton de la koncerto.

Difference in feel:

por ne forgesi la daton – in order not to forget the date; a bit more compact, uses the infinitive forgesi.
por ke mi ne forgesu la daton – so that I don’t forget the date; slightly more explicit and formal-sounding.

Both are grammatical because the subject of the main verb (mi metas) and of the “forgetting” is the same person (mi). When the subject is different, you must use por ke + -u:

Mi metas noton sur la tablon, por ke vi ne forgesu la daton.
(I put a note on the table so that you don’t forget the date.)

Why is there a comma before por ke?

The comma separates the main clause from the purpose clause:

Main clause: Hejme mi metas la bileton en la libron
Purpose clause: por ke mi ne forgesu la daton de la koncerto

In Esperanto, commas are commonly used to separate such clauses. It’s not absolutely obligatory in all styles, but it’s good, clear writing and very standard in cases like this.

Why is it daton with -n, and why use de la koncerto?

Daton has -n because it is the direct object of ne forgesu:

mi ne forgesu la daton – that I not forget the date (I must not forget what? the date.)

Then de la koncerto just specifies which date:

la dato de la koncerto = the date of the concert

So the structure is:

forgesi la daton (de la koncerto)
forget the date (of the concert)

Why doesn’t koncerto have -n in de la koncerto?

Because koncerto is not a direct object; it is inside a prepositional phrase with de.

General rule: prepositions in Esperanto (like de, en, sur, pri) take the basic form of the noun (no -n) unless you are using -n specifically to show direction (with some prepositions like en, sur, sub, etc.).

Here:

de la koncerto = of the concert (no movement, no direct object)
So we just use koncerto, not koncerton.

Why is it metas (present tense) and not metos (future), since I’m doing it to avoid a future forgetting?

Metas is present tense, and here it describes what you (habitually) do at home:

Hejme mi metas la bileton en la libron…
– At home I (usually) put the ticket in the book…

Even though the reason concerns the future (“so I won’t forget later”), the action of putting happens in the present (or as a general habit), so metas is right.

If you were talking about a specific future plan, you could say:

HejmE mi METOS la bileton en la libron, por ke mi ne forgesu la daton.
– At home I will put the ticket in the book, so that I don’t forget the date.

But in the given sentence, the simple metas fits well as a habit or a general description of what you do.