Cet adjectif est au féminin parce qu’il décrit Marie.

Because adjectif is a masculine singular noun, and it begins with a vowel sound.

French demonstratives work like this:

• ce = masculine singular before most consonant sounds
• cet = masculine singular before a vowel or silent h
• cette = feminine singular

So:

• ce livre = this book
• cet adjectif = this adjective
• cette phrase = this sentence

A common mistake is to choose cette because Marie is feminine, but cet agrees with adjectif, not with Marie.

It means this adjective.

Here:

• cet = this
• adjectif = adjective

So the sentence is talking about one specific adjective.

Est au féminin means is in the feminine form.

Literally:

• est = is
• au = in the / in a form of
• féminin = feminine

In grammar, French often uses:

• au masculin = in the masculine form
• au féminin = in the feminine form
• au singulier = in the singular
• au pluriel = in the plural

So cet adjectif est au féminin means the adjective has the feminine ending or feminine form.

Because in au féminin, féminin is part of a fixed grammatical expression.

You can think of it as short for something like:

• au genre féminin
• au masculin / au féminin

In grammar talk, le féminin and le masculin are treated like nouns naming grammatical categories. That is why you see féminin, not féminine, in this expression.

Because que becomes qu’ before a vowel sound.

This is called elision.

• parce que il would sound awkward in French
• so it becomes parce qu’il

The same thing happens in many other cases:

• je aime → j’aime
• le ami → l’ami
• que elle → qu’elle

Here, il refers to cet adjectif.

So:

• cet adjectif = the adjective
• il décrit Marie = it describes Marie

Even though il often means he, it can also mean it when referring to a masculine noun.

Since adjectif is masculine, French uses il for it.

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

That means the adjective changes form to match:

• gender: masculine or feminine
• number: singular or plural

Since Marie is feminine and singular, the adjective describing her must normally be feminine singular too.

For example:

• Marie est petite.
• Paul est petit.

The adjective changes from petit to petite because it agrees with the person described.

No. Cet agrees with adjectif, not with Marie.

That is an important distinction:

• cet describes adjectif
• the adjective mentioned in the sentence describes Marie

So:

• cet adjectif is masculine because adjectif is a masculine noun
• the adjective itself is feminine because it describes Marie

French can have different agreement patterns in the same sentence depending on what each word refers to.

Because Marie is a proper name, and proper names in French usually do not take an article.

So you normally say:

• Marie
• Paul
• Sophie

not:

• la Marie
• le Paul

There are a few special regional or stylistic exceptions, but in standard French, names usually appear without an article.

Because décrire takes a direct object in French.

So you say:

• décrire quelqu’un
• décrire quelque chose

That means:

• il décrit Marie = it describes Marie

You do not need a preposition like à or de here.

Yes. Décrit is the third-person singular present form of décrire.

Conjugation:

• je décris
• tu décris
• il / elle décrit
• nous décrivons
• vous décrivez
• ils / elles décrivent

So in il décrit Marie, the subject is il, and the verb is décrit.

Yes. For example:

• Cet adjectif est au masculin parce qu’il décrit Paul.

That means the same structure is being used, but now the adjective is masculine because it describes Paul, who is masculine.

So the contrast is:

• ... au féminin ... décrit Marie
• ... au masculin ... décrit Paul

Not quite.

• féminin by itself means feminine
• au féminin means in the feminine form

So:

• un adjectif féminin = a feminine adjective
• cet adjectif est au féminin = this adjective is in the feminine form

The second one sounds more like a grammar explanation about the form being used.

A simple pronunciation guide is:

Cet adjectif est au féminin parce qu’il décrit Marie
≈ seh tah-jek-teef eh toh fay-muh-nan parsk keel day-kree mah-ree

A few useful notes:

• cet adjectif links together smoothly
• qu’il sounds like keel
• the final consonants in many French words are not pronounced
• féminin ends with a nasal sound, not a clear n

This sentence is also a good example of how French flows through linking and elision.