Cette chanson est vraiment magnifique.

Because chanson is a feminine singular noun in French.

French demonstratives change form depending on gender and number:

• ce = masculine singular
• cet = masculine singular before a vowel sound
• cette = feminine singular
• ces = plural

So:

• ce livre = this book
• cet homme = this man
• cette chanson = this song
• ces chansons = these songs

Since chanson is feminine, you need cette.

In French, every noun has a grammatical gender, either masculine or feminine. Unfortunately, there is not always a perfect rule, so you often have to learn the gender together with the noun.

For this word, it is:

• une chanson = a song

That tells you it is feminine. A good habit is to memorize nouns with their article:

• une chanson
• un livre
• une voiture

That way, gender becomes easier to remember.

Because the sentence already has a clear subject: Cette chanson.

• Cette chanson est vraiment magnifique. = This song is really magnificent.

Here, cette chanson is the subject, so you use the verb être normally: est.

You use c’est more often when saying it is / this is before a noun, name, or general identification:

• C’est une belle chanson.
• C’est Marie.
• C’est magnifique.

But when the subject is explicitly stated, French usually uses normal subject + verb structure:

• Cette chanson est belle.
• Le film est intéressant.

Because vraiment is an adverb, and here it modifies the adjective magnifique.

In English, we also usually put really before the adjective:

• really beautiful
• really interesting
• really magnificent

French works the same way here:

• vraiment magnifique
• vraiment beau
• vraiment intéressant

So vraiment means really, and it is placed before the adjective it intensifies.

You might hear it sometimes, but vraiment magnifique sounds more natural.

Why? Because magnifique already expresses a strong idea, so très magnifique can feel a bit heavy or awkward. French often prefers:

• vraiment magnifique = really magnificent
• absolument magnifique = absolutely magnificent
• magnifique by itself

By contrast, très works very naturally with many adjectives:

• très beau
• très joli
• très bon

So vraiment magnifique is a very natural choice.

In this sentence, magnifique does not change in spelling.

Many French adjectives agree with the noun they describe, but some have the same form in masculine and feminine singular. Magnifique is one of them:

• un film magnifique
• une chanson magnifique

The spelling stays the same.
However, it would change in the plural:

• des films magnifiques
• des chansons magnifiques

So yes, it agrees grammatically, but here the masculine and feminine singular forms happen to look identical.

A careful pronunciation is approximately:

set sha(n)-so(n) eh vray-mahn ma-nyee-feek

A few useful notes:

• cette sounds like set
• chanson has a nasal sound; the on is not pronounced like English on
• est is pronounced like eh
• vraiment is pronounced roughly vray-mahn
• magnifique is roughly ma-nyee-feek

In natural speech, French rhythm matters a lot, so it flows smoothly:

Cette chanson est vraiment magnifique.

Yes, there can be one between est and vraiment, though it is not very strong or always emphasized in slow speech.

You may hear:

• est vraiment pronounced a bit like eh vr...

There is no liaison between cette and chanson, because cette ends in a pronounced t already, and chanson begins with ch.

There is also no special liaison in vraiment magnifique because vraiment does not end in a silent consonant that links in the usual way.

Because cette already functions as a determiner.

In French, you generally use one determiner, not two. So you say:

• cette chanson = this song
• la chanson = the song
• une chanson = a song

You do not say:

• la cette chanson ❌
• une cette chanson ❌

So cette replaces the article.

Yes, sometimes cette chanson can mean either this song or that song, depending on context.

French often uses the same demonstrative for both, and context tells you which one is meant. If you want to make the contrast clearer, French can add -ci or -là:

• cette chanson-ci = this song
• cette chanson-là = that song

But in everyday speech, people often just say cette chanson.

It is common, but it is definitely stronger and more expressive than something like belle.

Compare:

• Cette chanson est belle. = This song is beautiful.
• Cette chanson est très belle. = This song is very beautiful.
• Cette chanson est magnifique. = This song is magnificent/gorgeous.

So magnifique sounds enthusiastic and emotional. It is very natural if you really want to praise the song strongly.

Yes, that is possible, but it changes the tone slightly.

• Cette chanson est vraiment magnifique. = neutral, standard, natural
• Cette chanson est magnifique, vraiment. = more emphatic, like This song is magnificent, really

The original version is the most straightforward and common if you are simply describing the song.

Here is the breakdown:

• Cette = demonstrative determiner (this/that)
• chanson = noun (song)
• est = verb, 3rd person singular of être (is)
• vraiment = adverb (really)
• magnifique = adjective (magnificent)

So the structure is:

determiner + noun + verb + adverb + adjective

That is a very common and useful French sentence pattern.