How to form questions in French and which register should I use in different situations?

There are three ways to form a question in French, ranging from informal to very formal (mostly used in writing). The first way to ask a question and the most informal way is through simple intonation. An example: "Elle est italienne." (She is Italian.) would transform into "Elle est italienne?" (Is she Italian?). The only difference being our intonation at the end of the sentence. This is never used in writing.The second and perhaps most widely used in spoken French, neither informal nor overly formal, is to use "Est-ce que" at the start of the sentence. An example: "Est-ce qu'elle est italenne?" (Is she Italian?). Note that this cannot be combined with inversion - the last way to form a question.Lastly, inversion is the most formal way to form a question and as such is most commonly used in writing. Using the same example, we would say "Est-elle italienne?" (Is she Italian?). Referring back to "Est-ce que", we can never combine the two: "Est-ce qu'est-elle italienne?" is wrong.