Kiam estas multe da humido, radikoj putras kaj ŝimo kreskas.

Kiam means when. It introduces a time clause: Kiam estas multe da humido = When there is a lot of moisture.

So the sentence has two parts:

• Kiam estas multe da humido = the condition in time
• radikoj putras kaj ŝimo kreskas = what happens then

In Esperanto, estas can be used impersonally, like English there is or it is in some expressions.

So:

• estas multe da humido literally looks like is much moisture
• but naturally it means there is a lot of moisture

You do not need a dummy subject like English there.

Multe da humido is the normal way to say a lot of moisture.

Here is the pattern:

• multe da + noun = a lot of ...
• iom da + noun = some ...
• sufiĉe da + noun = enough ...

So:

• multe da humido = a lot of moisture

You may sometimes see multa humido, but multe da humido is the more standard quantity expression here.

Esperanto uses da after words of quantity, amount, measure, or degree.

So:

• multe da humido = a lot of moisture
• glaso da akvo = a glass of water
• kilogramo da rizo = a kilogram of rice

De usually shows possession, origin, or a more general of relation, not a quantity relationship.

Humido is a mass noun here, like moisture in English. It is not being counted as separate items.

So:

• humido = moisture
• not humidoj, because we are not talking about several separate moistures

This is very normal with substances, materials, and abstract things.

The ending -oj marks the plural.

• radiko = root
• radikoj = roots

So radikoj putras means roots rot or the roots are rotting.

Because the sentence is making a general statement, not talking about one specific set of roots or one specific mold already identified.

So:

• radikoj putras = roots rot / roots start rotting
• ŝimo kreskas = mold grows

If you said la radikoj, that would usually mean the roots of some specific plant already being discussed.

The ending -as marks the present tense in Esperanto.

• putras = rot / are rotting
• kreskas = grows / is growing

Esperanto verbs do not change according to the subject. So the same form works for singular and plural:

• radiko putras = a root rots
• radikoj putras = roots rot

Because there is no direct object here.

• radikoj is the subject of putras
• ŝimo is the subject of kreskas
• humido comes after da, not as a direct object

The accusative -n is mainly used for direct objects and for some movement expressions. Neither applies here.

Not exactly without changing the nuance.

• putras = rot / are rotting
• putriĝas = become rotten / start to rot

Putras simply describes the process or state.
Putriĝas adds the idea of becoming that way.

In this sentence, putras is the simpler and more neutral choice.

Yes. It is a natural Esperanto expression.

• ŝimo = mold
• kreskas = grows

So ŝimo kreskas means mold develops or grows. That matches how English also talks about mold.

Ŝ is pronounced like English sh.

So:

• ŝimo sounds roughly like SHEE-mo

A few other useful letter notes from the sentence:

• kiam = roughly KEE-am
• radikoj = ra-DEE-koy
• putras = POO-tras
• kreskas = KRES-kas

The comma separates the introductory subordinate clause from the main clause.

• Kiam estas multe da humido, = When there is a lot of moisture,
• radikoj putras kaj ŝimo kreskas. = roots rot and mold grows.

This is also very similar to English punctuation in sentences that begin with when clauses.

Not completely. Esperanto word order is fairly flexible, because endings carry much of the grammar.

This sentence is the most straightforward order:

• time clause first
• then the main statement

You could change the order for emphasis, for example:

• Radikoj putras kaj ŝimo kreskas, kiam estas multe da humido.

That still means essentially the same thing. The original version is just the most natural if you want to start with the condition.