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

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Breakdown of Kiam pluvo falas sur la tero, la ĝardeno ŝajnas pli verda ol en vintro.

• en: in
• ĝardeno: the garden
• sur: on

Questions & Answers about Kiam pluvo falas sur la tero, la ĝardeno ŝajnas pli verda ol en vintro.

Why does the sentence use “pluvo falas” instead of just “pluvas”?

Esperanto has both:

• pluvi = to rain (verb, impersonal)
• pluvo = rain (noun)
• fali = to fall

So:

• Pluvas. = It is raining. (no explicit subject)
• Pluvo falas (sur la teron). = Rain is falling (onto the ground).

In the sentence, “pluvo falas sur la tero” literally means “rain falls on the ground” and gives a more concrete, visual image of drops falling.

You could also say:

• Kiam pluvas, la ĝardeno ŝajnas pli verda ol en vintro.

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.