Multe da humido restas en la substrato.

Both can work, but they are structured differently.

• multe da humido = a lot of moisture
• multa humido = much moisture

In multe da humido, multe is a quantity word, and da links that quantity to the noun. This is a very common Esperanto pattern:

• multe da akvo
• iom da tempo
• sufiĉe da mono

Using multa humido is also possible, but multe da humido often sounds a bit more like you are focusing on the amount itself.

da is used after words of quantity, measure, or amount to show what the amount is of.

So:

• multe da humido = a lot of moisture
• literally, something like much amount of moisture

This is different from de, which usually shows things like possession, origin, or relation:

• la pordo de la domo = the door of the house
• glaso da akvo = a glass of water

A good rule of thumb is:

• use da after quantity/measure words
• use de for ordinary of

Because in the pattern quantity word + da + noun, Esperanto normally uses the adverb-like form:

• multe da
• iom da
• tro da
• sufiĉe da

Here, multe is not directly acting like an adjective on humido. Instead, it expresses an amount, and da connects that amount to humido.

So multe da humido is a fixed and very natural pattern in Esperanto.

restas is the present-tense form of resti, meaning to remain or to stay.

In Esperanto, all present-tense verbs end in -as:

• mi restas
• vi restas
• ĝi restas
• ili restas

So the form does not change depending on the subject, unlike in English.

Here, restas means that the moisture is still there or remains in the substrate.

Because humido is not a direct object here. It is the thing that remains.

In Esperanto, the accusative -n is mainly used for:

• direct objects
• direction toward a place in some cases

Here, humido is part of the subject idea: multe da humido.

So no -n is needed.

Because en la substrato describes location, not motion.

• en la substrato = in the substrate
• en la substraton would suggest motion into the substrate

Since the sentence says where the moisture remains, and not that something moves into it, the normal form is en la substrato.

la shows that we mean a specific substrate, not substrate in general.

So:

• en la substrato = in the substrate / in the particular substrate being discussed
• en substrato would sound more general, more like in substrate as a material or category

Esperanto uses la somewhat similarly to English the. There is no indefinite article like a/an, but the definite article la is used when the thing is identifiable from context.

Yes. Esperanto word order is fairly flexible, because grammar is shown mostly by endings and function words.

This sentence could also be:

• En la substrato restas multe da humido.

That version puts more focus on where the moisture remains.

The original:

• Multe da humido restas en la substrato.

puts the focus a bit more naturally on how much moisture remains.

Both are grammatical.

Normally, humido is treated as an uncountable or mass noun, like moisture in English.

So you usually talk about it in terms of amount:

• multe da humido
• iom da humido
• tro da humido

If you want something countable, you would use a unit or container word, for example:

• du gutoj da akvo = two drops of water
• alta nivelo de humido = a high level of moisture

So in this sentence, humido is being treated as a substance, not as separate countable items.

You could, but the meaning would change slightly.

• estas en la substrato = is in the substrate
• restas en la substrato = remains in the substrate

restas implies that some moisture is still there, possibly after something else has happened, such as drying, draining, or absorption.

So restas gives a stronger sense of still being left.

Yes. substrato is a valid Esperanto word, and it is especially common in technical or scientific contexts.

Depending on context, it can mean things like:

• substrate
• growing medium
• base layer
• underlying material

It is built in a very Esperanto-friendly way and looks similar to related international words, so it is easy to recognize if you know English or other European languages.