Learn Hardcore Esperanto: Vespere la patro diras, ke lia tago estis pli longa ol la tago de la patrino. - In the evening the father says that his day was longer than the mother's day.

The root is vesper- (evening).
-o would make it a noun: vespero = an evening / the evening.
-a would make it an adjective: vespera = evening (as an adjective), e.g. vespera manĝo = evening meal.
-e makes it an adverb: vespere = in the evening (describes when something happens).

So Vespere la patro diras... = In the evening the father says...

Could I say En la vespero instead of vespere? Is there a difference?

Yes, you can say En la vespero la patro diras..., and it is grammatically correct. The difference is mostly nuance and style:

Vespere is a simple adverb; it usually suggests a regular or typical time: in the evenings / in the evening (generally).
En la vespero points a bit more to a particular evening or to the evening as a more concrete time period: in the evening (this specific day).

In many contexts they are interchangeable, and Esperanto speakers frequently use vespere for this kind of time expression because it’s shorter and very natural.

Why do we say la patro and la patrino, but not la vespere or la lia tago?

In Esperanto:

la is the definite article “the”.
It is used before nouns when we mean a specific, identifiable thing or person:
- la patro = the father
- la patrino = the mother
- la tago = the day

We do not use la:

Before adverbs: vespere is an adverb, so no article: not la vespere.
Before possessive adjectives like lia, ŝia, mia, etc.:
- lia tago = his day, not la lia tago.
- The possessive (lia) already makes the noun specific, so we don’t add la.

Why is it diras (present tense) but estis (past tense)? Can it be diris?

Both diras and diris are possible, but they say slightly different things:

diras = says (present tense)
diris = said (past tense)
estis = was (past tense of esti)

Your sentence:

Vespere la patro diras, ke lia tago estis pli longa...
- Literally: In the evening the father says that his day was longer than the mother’s day.

Here:

The saying is presented in the present tense (diras), perhaps as a habitual event (he says this in the evening), or as a kind of “narrative present”.
The day being longer clearly refers to that same day just ending, so we use past tense: estis.

If you want everything clearly in the past, you can say:

Vespere la patro diris, ke lia tago estis pli longa ol la tago de la patrino.
= In the evening the father said that his day had been longer than the mother’s day.

Esperanto does not require tense changes inside a ke-clause the way English sometimes does; you choose the tense that fits the time of the action.

What is the role of ke in diras, ke lia tago estis...? Can it be omitted like English “that”?

Ke is a conjunction that introduces a content clause (a “that-clause”):

la patro diras, ke... = the father says that...

In Esperanto, ke is not usually omitted. English allows:

He says that his day was longer...
He says his day was longer... (no that)

Esperanto, however, normally must keep ke:

✅ Li diras, ke lia tago estis pli longa...
❌ Li diras lia tago estis pli longa...

So ke clearly marks the beginning of the clause that is being said, known, thought, etc.

Why is it lia tago and not sia tago? I thought si- was used for “his own”.

This is a classic Esperanto point.

lia = his (referring to some male person, not automatically the subject)
sia = his own / her own / its own / their own, but only when it refers back to the subject of the same clause.

In your sentence:

Main clause subject: la patro
Subordinate clause: ke lia tago estis pli longa...
- Subject of this clause: lia tago (= his day)

Because lia is inside the ke-clause, and in that clause the subject is tago, sia would point back to tago, not to la patro. That doesn’t make sense.

So we use lia to mean “his (the father’s) day”.

If we rewrote the sentence so that patro is the subject of the same clause as the possessive, then sia would be correct. For example:

La patro opinias sian tagon pli longan ol la tago de la patrino.
(Here sian refers back to la patro, the subject of the same clause.)

Could I say la tago de li instead of lia tago?

You can say la tago de li, and it is grammatically correct: literally the day of him.

However, in normal Esperanto style:

lia tago is much more natural and preferred.
la tago de li sounds more heavy or emphatic, and is used much less often.

Use lia tago for ordinary possessive meaning his day.

How does the comparison pli longa ol la tago de la patrino work? Why pli and ol?

The structure for comparatives in Esperanto is:

pli + adjective + ol = more ... than ...

So:

longa = long
pli longa = longer / more long
ol = than

Putting it together:

lia tago estis pli longa ol la tago de la patrino
= his day was longer than the mother’s day.

You can also omit the repeated parts that are clearly understood, just as in English:

Fully explicit: lia tago estis pli longa ol la tago de la patrino estis longa
Natural, shorter form: lia tago estis pli longa ol la tago de la patrino
(We drop the second estis longa because it’s obvious.)

Why is it pli longa, not pli long? What does the -a do here?

In Esperanto:

Adjectives end in -a.
Adverbs end in -e.
Nouns end in -o.

Longa is an adjective meaning long, and it must keep the -a even when used with pli:

longa = long
pli longa = longer / more long
tre longa = very long

You cannot drop the -a:

❌ pli long (incorrect)
✅ pli longa (correct)

Similarly:

bela → pli bela
granda → pli granda

Why don’t tago or tagon take -n here? I thought direct objects take -n.

In Esperanto, -n (accusative) is used mainly for:

Direct objects of verbs.
Indicating direction after some prepositions or without a preposition.

In your sentence:

lia tago is the subject of estis:
- lia tago (subject) estis (verb) pli longa (predicate adjective).
la tago de la patrino is just part of a comparison introduced by ol:
- pli longa ol la tago de la patrino

Neither of these is a direct object, so they do not get -n:

✅ lia tago estis pli longa ol la tago de la patrino
❌ lia tagon estis pli longa... (wrong)
❌ ...ol la tagon de la patrino (wrong in this sentence)

Can vespere be placed somewhere else in the sentence, like in English?

Yes, Esperanto word order is fairly flexible, especially for adverbs of time such as vespere.

Some natural variants:

Vespere la patro diras, ke lia tago estis pli longa ol la tago de la patrino.
La patro vespere diras, ke lia tago estis pli longa ol la tago de la patrino.
La patro diras vespere, ke lia tago estis pli longa ol la tago de la patrino.

All are grammatical. The differences are in emphasis and rhythm:

Vespere la patro diras... puts a little emphasis on when first.
La patro vespere diras... highlights la patro first, then adds in the evening as extra info about him.
La patro diras vespere, ke... emphasizes the act of saying and then adds when.

But all are quite acceptable.

Could we say ...ke lia tago estis pli longa ol tiu de la patrino instead of repeating tago?

Yes, that is a very natural alternative:

...ke lia tago estis pli longa ol tiu de la patrino.

Here:

tiu = that one (a demonstrative pronoun).
ol tiu de la patrino = than that (one) of the mother = than the mother’s.

This avoids repeating tago, just like English:

his day was longer than the mother’s. (we omit day)

Both versions are fine:

...pli longa ol la tago de la patrino
...pli longa ol tiu de la patrino

The first is slightly more explicit; the second is a bit more compact.