Cette affiche ancienne est mon coup de cœur.

Because affiche is a feminine singular noun.

French demonstratives agree with the noun:

• ce = masculine singular before a consonant
• cet = masculine singular before a vowel or silent h
• cette = feminine singular
• ces = plural

So:

• ce livre = this book
• cet arbre = this tree
• cette affiche = this poster

Even though affiche starts with a vowel, it is still feminine, so you must use cette.

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

Here, ancienne describes affiche, and affiche is:

• feminine
• singular

So the adjective must also be feminine singular:

• masculine: ancien
• feminine: ancienne

This is the same kind of agreement you see in:

• un livre ancien
• une affiche ancienne

In French, many adjectives come after the noun, unlike in English.

So French often says:

• une maison blanche = a white house
• un livre intéressant = an interesting book
• une affiche ancienne = an old/antique poster

Also, ancien / ancienne is a very useful adjective because its meaning can change depending on position.

• after the noun: old, ancient, antique
• before the noun: former

So in your sentence, affiche ancienne means an old poster, not a former poster.

Yes, at least in principle.

With ancien / ancienne, position matters:

• une affiche ancienne = an old / antique poster
• une ancienne affiche = a former poster

The second version is grammatically possible, but in real life it sounds odd because a poster is not usually something that is former in the same way a job, house, or teacher can be.

Compare:

• mon ancienne école = my former school
• une table ancienne = an antique table

So in your sentence, affiche ancienne is the natural choice.

Because the possessive adjective agrees with the noun being possessed, not with some other noun in the sentence.

Here, the thing that is mine is coup:

• un coup = masculine

So you say:

• mon coup de cœur

You do not choose mon / ma based on affiche. Even though affiche is feminine, it is not the noun after the possessive.

Compare:

• ma voiture = my car
• mon livre = my book
• mon coup de cœur = my favorite / my special favorite

It is an idiomatic expression. You usually should not translate it word for word.

Literally, it is something like a strike to the heart, but in actual French it means something like:

• a favorite
• a personal favorite
• something I really love
• something I fell for immediately

It often has a warm, enthusiastic feeling. It can suggest a strong personal liking, sometimes almost like love at first sight for an object, artwork, book, film, place, and so on.

So Cette affiche ancienne est mon coup de cœur means something like:

• This old poster is my favorite
• This antique poster is my personal favorite
• This old poster is the one I really love

Because coup de cœur is a fixed expression built as noun + de + noun.

In many French expressions like this, the second noun appears without an article:

• coup de foudre
• pomme de terre
• salle de bain

So coup de cœur is just the standard form of the expression. You learn it as a whole phrase.

It is better not to analyze it too literally every time. Think of coup de cœur as one vocabulary item.

Not exactly.

Both can relate to old, but they do not always feel the same.

• vieille is the more everyday word for old
• ancienne after the noun often suggests old from an earlier period, antique, historical, or vintage

So:

• une vieille affiche = an old poster
• une affiche ancienne = an old/antique poster, often with a more refined or historical feel

In your sentence, affiche ancienne sounds a bit more like a collector’s item or something with period charm.

Not very naturally.

A word-for-word translation would sound strange because coup de cœur is idiomatic. English usually uses a more natural equivalent.

Good translations would be:

• This old poster is my favorite.
• This antique poster is my personal favorite.
• This vintage poster is the one I love most.

So the grammar can be analyzed word by word, but the full meaning should be understood idiomatically.

A rough pronunciation guide is:

set a-feesh ahn-syen eh mon koo duh kur

A few useful notes:

• cette sounds like set
• affiche sounds like a-feesh
• ancienne has two syllables after the initial nasal sound: roughly ahn-syen
• coup sounds like koo
• cœur has no exact English equivalent; it is something like kur with a French vowel

Also, in normal speech, the words flow together smoothly:

• cette affiche
• mon coup de cœur

So it sounds connected, not chopped into separate words.