Le substrat reste mouillé.

Because rester means to stay / to remain, while être just means to be.

So:

• Le substrat est mouillé = The substrate is wet
• Le substrat reste mouillé = The substrate remains wet / stays wet

The sentence with reste emphasizes that it continues to be wet, rather than simply describing its condition.

Reste is the third-person singular present tense form of rester.

The present tense forms are:

• je reste
• tu restes
• il / elle / on reste
• nous restons
• vous restez
• ils / elles restent

Since le substrat is singular and behaves like it, French uses reste.

Le is the masculine singular definite article, meaning the.

So:

• le substrat = the substrate

French uses articles more regularly than English does, so where English sometimes drops the, French often keeps it.

You mainly know it by learning it with its article:

• le substrat

That tells you it is masculine singular. In French, noun gender is something you usually need to memorize along with the noun itself.

Because substrat is masculine singular, the adjective also appears in the masculine singular form: mouillé.

Because adjectives in French usually agree with the noun they describe.

Here, the noun is:

• le substrat → masculine singular

So the adjective must also be masculine singular:

• mouillé

Compare:

• le substrat mouillé = masculine singular
• la surface mouillée = feminine singular
• les substrats mouillés = masculine plural
• les surfaces mouillées = feminine plural

In this sentence, it functions as an adjective meaning wet.

Historically, it is also the past participle of mouiller (to wet / to moisten), but in rester mouillé, it is best understood as an adjective describing the state of the substrate.

So the structure is:

• subject: le substrat
• verb: reste
• adjective: mouillé

Both can relate to moisture, but they are not always exactly the same.

• mouillé = wet
• humide = damp / humid / moist

So mouillé usually suggests more obvious wetness, while humide can sound milder.

For example:

• Le sol est mouillé = The ground is wet
• L’air est humide = The air is humid
• Le tissu est humide = The fabric is damp

In your sentence, mouillé suggests the substrate is actually wet, not just slightly damp.

A careful approximate pronunciation is:

luh sub-stra rehst moo-yay

More naturally in IPA:

/lə sypstʁa ʁɛst muje/

A few pronunciation notes:

• Le is usually a light luh
• substrat ends with a pronounced a
• reste sounds like rest
• mouillé sounds roughly like moo-yay

The ll in mouillé does not sound like English l. It becomes a y-like sound.

Not all of them.

• In reste, the final -e is not pronounced.
• In mouillé, the final -é is pronounced as ay.
• In substrat, the final -t is usually silent.

So the sentence does not sound like every letter is spoken.

Yes. Both are common translations.

In this sentence:

• Le substrat reste mouillé = The substrate stays wet
• Le substrat reste mouillé = The substrate remains wet

Remain is often a little more formal in English, while stay is more everyday, but both fit the French sentence.

Yes, substrat is a technical noun, much like substrate in English.

It is commonly used in scientific, biological, agricultural, and materials-related contexts. Depending on context, it can refer to a growing medium, a base layer, or a material on which something develops.

So for an English speaker, this is a fairly transparent vocabulary item, but you still need to remember its French gender:

• le substrat

No, it follows a very standard French pattern:

• Le substrat = subject
• reste = verb
• mouillé = adjective describing the subject

So it works like:

• Le ciel reste gris = The sky remains gray
• La porte reste ouverte = The door remains open

This pattern is very common with verbs like être, sembler, devenir, and rester followed by an adjective.