La moisissure couvre la tache d’humidité.

Because there are two different feminine singular nouns:

• la moisissure = the mold / mildew
• la tache = the stain / mark

In French, nouns usually need an article, and the article must match the noun’s gender and number. Here, both nouns are feminine singular, so both take la.

This is because of elision.

The preposition de becomes d’ before a word that starts with a vowel or a silent h:

• de + humidité → d’humidité

French does this to make pronunciation smoother. Since humidité begins with a silent h, you must say d’humidité, not de humidité.

It is a noun phrase made of:

• tache = stain / spot
• d’humidité = of dampness / caused by moisture

So literally it is something like a moisture stain or a damp patch.

This pattern is very common in French:

• une bouteille d’eau = a bottle of water
• une table de cuisine = a kitchen table
• une tache d’huile = an oil stain

So de / d’ often links one noun to another.

Because the subject is singular: la moisissure.

The verb couvrir must agree with its subject:

• je couvre
• tu couvres
• il / elle couvre
• nous couvrons
• vous couvrez
• ils / elles couvrent

Here, la moisissure is treated as she/it, singular, so the correct form is couvre.

Yes, in this sentence it is very similar to English:

• La moisissure = subject
• couvre = verb
• la tache d’humidité = direct object

So it follows a straightforward subject + verb + object pattern:

The mold covers the damp stain.

That makes this sentence relatively easy for English speakers structurally.

Because couvrir is a direct transitive verb in French, just like to cover in English.

You say:

• couvrir quelque chose = to cover something

So:

• La moisissure couvre la tache d’humidité
  = The mold covers the damp stain.

You do not need a preposition like sur here.

No. They are different words, and this is a very common point of confusion.

• tache = stain, spot, mark
• tâche = task, job

So in your sentence, la tache d’humidité means the damp stain / moisture stain, not the humidity task.

The accent matters.

A rough pronunciation guide is:

mwah-zee-SYR

More precisely in IPA: /mwa.zi.syʁ/

A few helpful notes:

• moi sounds like mwa
• the s between vowels sounds like z
• u in sure is the French u, which English speakers often find difficult
• the final r is the French guttural r

You do not pronounce it the way an English speaker might guess from spelling.

No. The h in humidité is silent.

That is why:

• you say d’humidité
• you link smoothly from d’ into humidité

So it sounds approximately like:

yoo-mee-dee-TAY

The stress pattern is also different from English; French stress usually falls at the end of the phrase, not strongly on earlier syllables.

Yes, depending on context, moisissure can refer broadly to mold-like fungal growth, and in English the best translation may be mold, mildew, or mould in British spelling.

In many learning contexts, mold is the simplest general translation. The exact English choice depends on the situation, but the French grammar of the sentence does not change.

In this sentence, you can tell from the articles and form:

• la moisissure → feminine
• la tache → feminine
• d’humidité → humidité is also feminine; if it had an article by itself, it would be l’humidité

There is not always a perfect rule for guessing gender, so it is best to learn nouns together with their article:

• la moisissure
• la tache
• l’humidité

That will help you build correct sentences more naturally.