Learn Hardcore Esperanto: Kiam pluvo falas sur la tero, la ĝardeno ŝajnas pli verda ol en vintro. - When rain falls on the ground, the garden seems greener than in winter.

That is perfectly correct and probably more common in everyday speech.
“Pluvo falas” just emphasizes the physical act of rain falling rather than using the impersonal weather verb.

Why is “kiam” used here and not “se”?

kiam = when (talking about time)
se = if (talking about condition)

The sentence describes what happens whenever it rains, i.e. at that time:

Kiam pluvo falas sur la tero… = When rain falls on the ground…

Using se would shift the focus to a condition:

Se pluvo falas sur la tero… = If rain falls on the ground…

That sounds more hypothetical, as if rain might or might not happen.
With “kiam”, the idea is more like a general rule: each time it rains, the garden seems greener.

Why do we say “la ĝardeno” and “la tero”, but just “pluvo” without “la”?

Esperanto uses la for definite things, much like English “the”.

la ĝardeno – the garden
We are talking about one particular, identifiable garden (maybe the one you see from your window), so la is natural.
la tero – the ground / the earth
Here tero is a specific “ground” under the rain, not some vague amount of soil.
It’s similar to English: we say “on the ground”, not “on ground”.
pluvo – rain (in general)
Pluvo is a mass noun here, referring to rain as a substance or phenomenon in general. In that use, Esperanto usually omits the article, like:
- Akvo estas malseka. – Water is wet.
- Pluvo falas sur la tero. – Rain falls on the ground.

You could say “la pluvo” if you mean a specific, already known rainfall (e.g. “The rain that started a minute ago is falling…”), but here it’s clearly meant in general.

Why is the preposition “sur” used in “sur la tero”, and what exactly does it mean?

sur means “on” or “upon”, and depending on context it can also correspond to “onto” in English.

So:

sur la tero = on the ground / upon the ground

It tells us where the rain ends up, on which surface it falls.

Other possible prepositions, like al (“to/toward”), would be less natural here:

al la tero literally means “toward the earth/ground” and doesn’t express “onto the surface” as clearly as sur.

And about word choice:

tero = earth, ground, the Earth (planet), or soil in a general sense.
grundo = the soil you garden in; more “garden soil / ground” as material.

You could say “pluvo falas sur la grundon”, but “sur la tero” is broader and fits well with the idea of rain falling on the ground outside.

Shouldn’t it be “sur la teron” with -n for direction, since the rain is falling onto the ground?

Esperanto has the so‑called -n of direction:

sur la tablo = on the table (location)
sur la tablon = onto the table (movement to there)

With fali (to fall), both patterns are seen:

Li falis sur la planko. – "He fell on the floor." (focus on where he ended up)
Li falis sur la plankon. – "He fell onto the floor." (slightly more emphasis on the movement)

So in your sentence, both are grammatically possible:

Pluvo falas sur la tero.
Pluvo falas sur la teron.

The version without -n treats sur la tero more as the location involved in the situation (rain is falling, and its place is “on the ground”).
The version with -n emphasizes more strongly the movement onto the ground.

Many writers omit -n when the verb itself already clearly expresses movement (like fali), unless they need to avoid ambiguity. Others prefer to always use -n for motion. Both habits exist, and context makes the meaning clear in either case.

How does the comparative “pli verda ol en vintro” work, and why is it “verda” and not “verdan”?

Structure:

pli … ol … = more … than …
verda = green (adjective)

So:

pli verda = greener
pli verda ol en vintro = greener than (it is) in winter

Grammatically:

“verda” has -a because all adjectives in Esperanto end in -a.
There is no -n on verda because it is a predicate adjective, linked to the subject by a verb (ŝajnas):
- La ĝardeno ŝajnas pli verda. – The garden seems greener.
Predicate adjectives in Esperanto:
- agree only in number with the noun (singular/plural),
- do not normally take -n, even if the sentence has an object.

Examples:

La ĝardeno ŝajnas verda. – The garden seems green.
La ĝardenoj ŝajnas verdaj. – The gardens seem green.
La ĝardenoj ŝajnas pli verdaj ol hieraŭ. – The gardens seem greener than yesterday.

So “pli verda” correctly matches singular “la ĝardeno”, and the -n ending is simply not used in this role.

Why is “ŝajnas” used without “esti”, and what does “ŝajnas pli verda” literally mean?

ŝajni = to seem, to appear (to be)
esti = to be

ŝajni behaves like esti in that it connects the subject to an adjective or noun:

La ĝardeno ŝajnas verda. – The garden seems green.
La ĝardeno ŝajnas pli verda. – The garden seems greener.

You do not need “esti” here.
“ŝajnas pli verda” already means “seems to be greener”.

You can say:

La ĝardeno ŝajnas esti pli verda.

but it’s usually longer without adding much meaning. It can sound a bit more formal or slightly more “hedged” (as if you’re emphasizing that this is an appearance, not a fact), but in everyday language most speakers simply say “ŝajnas pli verda”.

What does “ol en vintro” mean exactly, and could we also say “ol vintre”?

ol = than (used in comparisons)
en vintro = in winter (as a season in general)

So:

pli verda ol en vintro = greener than (it is) in winter.

You’re comparing the garden’s greenness now (when it rains) to how green it generally is during winter.

About alternatives:

vintre is an adverb formed from vintra (wintry).
It means “in (the) winter, during wintertime”, like:
- Vintre estas malvarme. – In winter it’s cold.

You could say:

…ŝajnas pli verda ol vintre.

That is correct and quite idiomatic.
The original “ol en vintro” is slightly more transparent for learners and is also very common.
If you said “en la vintro”, it would suggest some particular winter, not winter in general.

Can the parts of the sentence be reordered, for example putting the garden first?

Yes. Esperanto word order is relatively flexible. Both of these are fine:

Kiam pluvo falas sur la tero, la ĝardeno ŝajnas pli verda ol en vintro.
La ĝardeno ŝajnas pli verda ol en vintro, kiam pluvo falas sur la tero.

They mean essentially the same thing. Differences:

Version 1 puts the “when it rains” idea first, so the time/condition is emphasized.
Version 2 starts with the garden being greener, and then adds under what circumstance that happens.

One small stylistic point: it’s usually good to keep “ol en vintro” close to “pli verda” so it’s clear that it belongs to the comparison (greener than in winter), not to the verb “falas”.

Why is there a comma, and why are both verbs in the present tense?

Comma

In Esperanto, it’s standard to use a comma between a main clause and a subordinate clause introduced by kiam, se, ke, etc., especially when the subordinate clause comes first:

Kiam pluvo falas sur la tero, la ĝardeno ŝajnas…

If the kiam‑clause comes second, many writers still keep the comma, but it’s less strictly required:

La ĝardeno ŝajnas pli verda ol en vintro, kiam pluvo falas sur la tero.

Present tense in both clauses

Both verbs are in the present tense:

falas – falls
ŝajnas – seems

Together, they express a general truth or habitual situation, just like in English:

When rain falls on the ground, the garden seems greener than in winter.

If you wanted to talk about a specific future case, you would use the future tense:

Kiam pluvos, la ĝardeno ŝajnos pli verda ol en vintro.
When it rains (in the future), the garden will seem greener than in winter.