Cette idée est folle.

Because idée is a feminine singular noun, and the demonstrative adjective must agree with it.

French uses:

• ce for masculine singular
• cette for feminine singular
• ces for plural

So:

• ce livre = this book
• cette idée = this idea
• ces idées = these ideas

Also, even though idée starts with a vowel, French still uses cette, not a special shortened form.

Noun gender in French is something you usually have to learn with each noun. Idée happens to be feminine, so you say:

• une idée
• cette idée
• une idée folle

A helpful clue is that many nouns ending in -ée are feminine, though not every single one.

Because adjectives usually have to agree with the noun they describe.

Since idée is feminine singular, the adjective must also be feminine singular:

• masculine singular: fou
• feminine singular: folle

So:

• un homme fou
• une idée folle

This is a common pattern in French: adjective endings often change depending on gender and number.

Some adjectives have an irregular feminine form. Instead of simply adding -e, fou changes spelling more noticeably:

• fou → folle
• mou → molle
• vieux → vieille

This happens because the feminine form reflects an older pronunciation pattern. It is something you mainly learn as a vocabulary pattern rather than trying to "build" it from a simple rule every time.

In French, most adjectives come after the noun, unlike English, where they usually come before it.

So French says:

• une idée folle literally: an idea crazy

This is normal French word order.

Some adjectives do come before the noun, but fou/folle usually comes after in a phrase like this.

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

So:

• je suis = I am
• tu es = you are
• il/elle/on est = he/she/one is

In Cette idée est folle, est means is.

Usually, est is pronounced like eh /ɛ/ in normal speech, and the final -st is not pronounced.

So:

• est sounds roughly like eh

But in est folle, you may hear a liaison before the vowel-like sound? No—because folle begins with f, not a vowel, so there is no liaison here.

A rough pronunciation of the whole sentence is:

• set ee-day eh fohl

A rough English-style pronunciation is:

set ee-day eh fohl

More carefully:

• cette = set
• idée = ee-day
• est = eh
• folle = fohl

A more IPA-style pronunciation would be roughly: /sɛ.t‿i.de ɛ fɔl/

One thing to notice: cette idée connects smoothly in speech.

Yes. In natural pronunciation, the final -t of cette is pronounced because the next word, idée, begins with a vowel sound.

So cette idée sounds like:

• set ee-day

This is very normal in French when one word ends in a consonant sound and the next begins with a vowel.

Yes. In everyday French, ce / cette / ces can mean either this or that, depending on context.

So cette idée could mean:

• this idea
• that idea

If French wants to be more explicit, it can use:

• cette idée-ci = this idea
• cette idée-là = that idea

But in most normal sentences, plain cette idée is enough.

Because cette already acts as a determiner.

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

• une idée = an idea
• cette idée = this/that idea

But not:

• cette une idée ❌

So cette is doing the job that this/that does in English.

Yes. Dingue is another very common way to say crazy.

So:

• Cette idée est folle
• Cette idée est dingue

Both are natural, but the tone can differ slightly depending on context:

• folle can sound a bit more standard or literary in some cases
• dingue is very common in everyday speech

Both are perfectly useful to know.

Yes. Like many adjectives, fou/folle can have different shades of meaning depending on context, such as:

• crazy
• mad
• wild
• insane

For example:

• une idée folle = a crazy / wild idea
• une passion folle = a crazy / intense passion
• une foule folle would suggest a mad or wild crowd

So the exact English translation depends on context, even though the basic idea stays similar.