Learn French: Cette fille est vraiment mignonne.

Most French adjectives just add an -e to make the feminine form. However, adjectives ending in -on (like the masculine mignon) follow a special rule: you must double the final consonant before adding the -e. That is why it becomes mignonne.

Does doubling the 'n' in mignonne change how the word sounds?

Yes, it completely changes the ending! In the masculine mignon, the -on is a nasal sound and you don't actually pronounce the letter 'n'. In the feminine mignonne, you drop the nasal sound and clearly pronounce the double 'n' (it sounds like 'meen-yon').

Why do we use cette here instead of ce?

Both ce and cette mean 'this', but they must match the gender of the noun they describe. Because fille (girl) is a feminine noun, we must use the feminine form cette. If we were talking about a boy (garçon), we would use the masculine ce (ce garçon).

Can mignonne be used to describe adults, or just children and animals?

While mignon and mignonne often mean 'cute' when talking about babies, pets, or small objects, you can definitely use them for adults too. When describing an adult, it generally means they are cute, sweet, or attractive in an endearing way.