Le substrat est pourri.

In French, nouns usually need an article more often than in English. So French says le substrat where English might just say the substrate or sometimes simply substrate, depending on context.

Here, le is the definite article meaning the. It also tells you that substrat is masculine singular.

It is masculine: le substrat.

That is why the sentence uses:

• le (not la)
• pourri (masculine form, not pourrie)

Est is the third-person singular form of the verb être (to be).

So:

• je suis = I am
• tu es = you are
• il / elle est = he / she is

Since le substrat is a singular subject, French uses est:

• Le substrat est pourri. = The substrate is rotten.

Because pourri agrees with substrat, and substrat is masculine singular.

Agreement works like this:

• masculine singular: pourri
• feminine singular: pourrie
• masculine plural: pourris
• feminine plural: pourries

Examples:

• Le fruit est pourri.
• La pomme est pourrie.
• Les fruits sont pourris.
• Les pommes sont pourries.

Yes. In Le substrat est pourri, pourri is an adjective describing the substrate.

The structure is:

• subject
  • être
    • adjective

So this is like:

• Le ciel est bleu.
• La soupe est chaude.
• Le substrat est pourri.

Yes, but it is slightly different in tone.

• Le substrat est pourri. is the more straightforward, neutral statement.
• Le substrat, c'est pourri. is more conversational and can sound more emphatic, like As for the substrate, it’s rotten or The substrate—it's rotten.

For a simple basic sentence, Le substrat est pourri is the safest version.

A simple approximation for an English speaker is:

luh sub-stra eh poo-ree

A few notes:

• le sounds like luh
• substrat ends with a silent t, so roughly sub-stra
• est is pronounced like eh
• pourri is roughly poo-ree, but with a French r

In normal speech, le may sound very light, and the whole sentence flows together smoothly.

Yes.

In careful standard pronunciation:

• the final t in substrat is silent
• the final t in est is silent

So:

• substrat sounds roughly like substra
• est sounds like eh

The word pourri does pronounce its final i sound.

Yes.

Literally, pourri means something like rotten, decayed, or gone bad.

It can also be used figuratively in informal French to mean something like:

• lousy
• awful
• terrible
• corrupt (in some contexts)

Examples:

• Un fruit pourri = a rotten fruit
• Une ambiance pourrie = a lousy atmosphere
• Un système pourri = a rotten/corrupt system

In your sentence, it is most naturally understood literally.

You would change the article, noun, verb, and adjective as needed:

• Les substrats sont pourris. = The substrates are rotten.

Changes:

• le → les
• substrat → substrats
• est → sont
• pourri → pourris

Usually, no. In this meaning, it normally comes after the noun when used directly with it.

So you would say:

• un substrat pourri = a rotten substrate

Not normally:

• un pourri substrat

And in your sentence, it appears after être, which is also completely normal:

• Le substrat est pourri.

You say:

Le substrat n'est pas pourri.

This uses the standard French negation:

• ne ... pas around the verb

So:

• est → n'est pas

In informal spoken French, people often drop ne, but in standard written French you should keep it:

• standard: Le substrat n'est pas pourri.
• informal speech: Le substrat est pas pourri.

It is a very common everyday word. It is not vulgar, but it can sound quite direct.

For physical decay, it is perfectly normal:

• Le bois est pourri.
• Le fruit est pourri.

In figurative uses, it can sound more expressive or negative:

• une journée pourrie = a crappy/lousy day

So in your sentence, it sounds natural and direct, not especially formal.