Learn Hardcore Esperanto: Post la vespermanĝo mi legas libron sur la lito.

libron has -n because it is what I am reading – it is the direct object of legas.
sur la lito is a prepositional phrase showing location. Words after a preposition (like sur, en, kun, etc.) normally do not take -n, unless you want to show direction (movement toward).

So:

Mi legas libron. – I read a book. (libron = direct object)
Mi sidas sur la lito. – I sit on the bed. (lito is in a phrase with sur, so no -n)

Why is there la in la vespermanĝo, but no word like “a” before libron?

Esperanto has only one article, la, which means “the”.
There is no separate word for “a/an”. If there is no la, it is usually understood as “a/an” or “some”, depending on context.

la vespermanĝo = “the dinner” (a specific dinner we know about, e.g., tonight’s dinner)
libron (no la) = “a book”

So:

Post la vespermanĝo – After the dinner / after dinner (that specific event tonight)
mi legas libron – I read a book (not a particular, previously identified book)

Could we say “Post vespermanĝo” without la? Would that change the meaning?

Yes, you can say Post vespermanĝo mi legas libron… and it’s grammatically correct.

Post la vespermanĝo sounds more like after the (particular) dinner just mentioned or understood from context.
Post vespermanĝo is a bit more general: after (having) dinner / after dinner time as a routine.

In everyday speech, both forms are possible; context usually makes the nuance clear.

What exactly does vespermanĝo mean, and how is that word formed?

vespermanĝo is a compound word, very typical in Esperanto.

vespero = evening
manĝo = a meal, or the act of eating (from the verb manĝi = to eat)
vespermanĝo = evening meal → dinner/supper

Esperanto freely builds new words like this, so once you know some roots (like mateno, tagmezo, manĝo), many combinations become easy to understand.

Why is the tense legas (present) used? In English I might say “I read”, “I am reading”, or even “I will read”.

Esperanto has one present tense, -as, and it roughly covers both English “I read” and “I am reading”.

Mi legas libron can mean:
- I am reading a book (right now).
- I read a book (as a regular activity).

If you clearly want the future, you use -os:

Post la vespermanĝo mi legos libron sur la lito. – After dinner I will read a book on the bed.

The context decides whether legas is understood as progressive or habitual.

Can I change the word order, for example to Mi legas libron sur la lito post la vespermanĝo?

Yes. Esperanto word order is fairly flexible, because roles are marked by endings (like -n) and by prepositions, not just by position.

All of these are grammatically correct and mean essentially the same thing:

Post la vespermanĝo mi legas libron sur la lito.
Mi legas libron sur la lito post la vespermanĝo.
Libron mi legas sur la lito post la vespermanĝo.

Changing the order can change emphasis (for example, putting Libron first emphasizes the book), but not the basic meaning.

What’s the difference between sur la lito and en la lito?

Literally:

sur la lito = on the bed (on top of it)
en la lito = in the bed (inside the covers / in bed)

In many languages there is an idiom “in bed” to mean “lying in bed, not up and dressed”; Esperanto usually uses:

en la lito = in bed (covered, tucked in, etc.)
sur la lito = physically on top of the bed (maybe sitting or lying on top of the covers).

So the original sentence suggests you are on the bed, not necessarily “in bed” for sleeping.

Is sur always “on (top of)”, or can it also mean “about” like in English “a book on history”?

The basic meaning of sur is “on (the surface of)”, as in the sentence here.

For “about / concerning (a topic)”, Esperanto normally uses pri:

Mi legas libron pri historio. – I am reading a book about history.

So:

libro sur la tablo – a book on the table (physically)
libro pri historio – a book about history (topic)

Why does post mean “after”? Isn’t there also malantaŭ for “behind/after”?

Both post and malantaŭ can relate to “behind”, but they are used differently:

post is very common for time:
- post la vespermanĝo – after dinner
  It can also be spatial: post la domo – behind the house (in a row, sequence, or relative position).
malantaŭ is more purely spatial:
- malantaŭ la domo – at the back of the house / behind the house (physically at the rear side).

For time, you normally use post, not malantaŭ.

How would I say “After dinner I am not reading a book on the bed”?

You just add ne (not) in front of the verb:

Post la vespermanĝo mi ne legas libron sur la lito.

The usual place for ne is directly before the verb. Everything else in the sentence stays the same.

How do I talk about more than one book or more than one bed in this kind of sentence?

Use -j for plural, and remember that -n (accusative) comes after the plural -j when needed.

Examples:

Mi legas librojn sur la lito. – I read books on the bed.
(libroj
- -n → librojn, plural direct object)
Mi legas libron sur la litoj. – I read a book on the beds. (several beds)
Mi legas librojn sur la litoj. – I read books on the beds.

So the endings combine like this:

singular: libro, libron
plural: libroj, librojn

Is there a special continuous form like “I am reading”, for example “mi estas leganta”?

Esperanto can form progressive-like structures with esti + -ant-:

Mi estas leganta libron. – literally I am being-reading a book.

However, in everyday Esperanto, this is usually unnecessary.
Mi legas libron is almost always enough, and context shows whether it means “I am reading” or “I read (habitually)”.

The forms with -ant-, -int-, -ont- exist and are correct, but they are used mainly when you really need to emphasize the aspect or to avoid ambiguity.