Cette bonne affaire est pour Paul.

Because affaire is a feminine singular noun.

French demonstrative adjectives change to match the noun:

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

Since affaire is feminine, you need cette: cette affaire.

Because adjectives usually agree with the noun they describe.

Here, bonne describes affaire, and affaire is feminine singular, so the adjective must also be feminine singular:

• masculine: bon
• feminine: bonne

So:

• un bon livre
• une bonne affaire

In this sentence, affaire is a noun meaning something like deal, business matter, or bargain, depending on context.

The expression bonne affaire is very common and often means a good deal or a bargain.

So even though affaire by itself can mean several things, bonne affaire works as a set phrase in many situations.

Actually, in bonne affaire, the adjective comes before the noun: bonne + affaire.

You are right that many French adjectives come after the noun, but some common adjectives often come before it, especially short and frequent ones like:

• bon / bonne
• petit / petite
• grand / grande
• jeune
• vieux / vieille

So bonne affaire is normal French word order.

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

• je suis
• tu es
• il/elle/on est

The subject is Cette bonne affaire, which is singular, so you use est.

Pour Paul means for Paul.

So the structure is:

Cette bonne affaire + est + pour Paul
subject + verb + prepositional phrase

Use pour when you mean for, intended for, meant for, or in favor of someone.

So est pour Paul means the deal is for Paul or intended for Paul.

À often means to, at, or shows possession/association in other structures, but it would not sound natural here for this meaning.

Compare:

• C’est pour Paul. = It’s for Paul.
• Je donne le livre à Paul. = I give the book to Paul.

So:

• pour = for
• à = to

Because Paul is a proper noun (a person’s name), and French normally does not use an article before a person’s first name.

So you say:

• pour Paul
• avec Marie
• chez Thomas

not:

• pour le Paul ❌

There are a few special regional or stylistic exceptions, but in standard French, no article is used here.

The sentence follows normal French word order:

Cette bonne affaire = subject
est = verb
pour Paul = complement

So it is simply:

Subject + verb + prepositional phrase

This is very similar to English word order in This good deal is for Paul, even though the exact French wording is different.

A natural pronunciation is roughly:

set bun na-fair ay poor pohl

A more French-like approximation:

• Cette → sounds like set
• bonne → sounds like bun
• affaire → sounds like a-fair
• est → sounds like ay or sometimes very light in connected speech
• pour → sounds like poor but with a more rounded French ou
• Paul → sounds like pohl

One useful point: there is often a liaison between bonne and affaire, so it can sound like:

bonne affaire → bon-naffaire

Yes. If you want to talk about more than one good deal, everything that can agree must change:

• Cette bonne affaire est pour Paul.
• Ces bonnes affaires sont pour Paul.

Changes:

• cette → ces
• bonne → bonnes
• affaire → affaires
• est → sont

This is a good example of agreement in French.

There are several natural ways:

• Cette bonne affaire est pour Paul ?
  Rising intonation in speech.

• Est-ce que cette bonne affaire est pour Paul ?
  Very common in everyday French.

• Cette bonne affaire est-elle pour Paul ?
  More formal inversion.

All three mean the same thing, but est-ce que is often the easiest and most useful pattern for learners.