Because idée is a feminine singular noun, and the demonstrative adjective must agree with it.
French uses:
So:
Also, even though idée starts with a vowel, French still uses cette, not a special shortened form.
Because adjectives usually have to agree with the noun they describe.
Since idée is feminine singular, the adjective must also be feminine singular:
So:
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:
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:
This is normal French word order.
Some adjectives do come before the noun, but fou/folle usually comes after in a phrase like this.
Usually, est is pronounced like eh /ɛ/ in normal speech, and the final -st is not pronounced.
So:
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:
Because cette already acts as a determiner.
In French, you normally use one determiner, not two. So you say:
But not:
So cette is doing the job that this/that does in English.
Yes. Dingue is another very common way to say crazy.
So:
Both are natural, but the tone can differ slightly depending on context:
Both are perfectly useful to know.
Yes. Like many adjectives, fou/folle can have different shades of meaning depending on context, such as:
For example:
So the exact English translation depends on context, even though the basic idea stays similar.