Mon neveu demande pourquoi le fiancé de Marie sourit autant, et ma nièce veut voir la bague.

Because French possessive words agree with the gender of the noun being possessed, not with the person who owns them.

• neveu = nephew, a masculine noun → mon neveu
• nièce = niece, a feminine noun → ma nièce

So even if the speaker is male or female, you still say:

• mon neveu
• ma nièce

They are the masculine and feminine forms of two related family words:

• neveu = nephew
• nièce = niece

A learner may notice that these words do not look very similar, so they usually just have to be memorized as separate vocabulary items.

Also, nièce has a grave accent: è.

French often uses a de + noun structure to make possession clearer.

• le fiancé de Marie = Marie’s fiancé / the fiancé of Marie

If the sentence said son fiancé, that could be ambiguous, because son can mean his or her depending on context. Using de Marie removes any doubt.

So le fiancé de Marie is a very natural way to say exactly whose fiancé is being talked about.

The difference is grammatical gender:

• fiancé = fiancé / engaged man
• fiancée = fiancée / engaged woman

In this sentence, le fiancé de Marie is masculine because it refers to a man.

You may also notice the spelling:

• masculine: fiancé
• feminine: fiancée

The feminine form adds another e.

Yes. Demander often means to ask.

Here, demande pourquoi... means asks why...

This is an indirect question:

• direct question: Pourquoi le fiancé de Marie sourit-il autant ?
• indirect question: Mon neveu demande pourquoi le fiancé de Marie sourit autant.

So after demande, the sentence keeps normal word order instead of question inversion.

Because sourit is the conjugated verb form, while sourire is the infinitive.

The infinitive is:

• sourire = to smile

But the subject here is le fiancé de Marie, which is he / it, so in the present tense you need:

• il sourit = he smiles / he is smiling

So:

• le fiancé de Marie sourit = Marie’s fiancé is smiling

In this sentence, autant means something like so much or that much.

• sourit autant = smiles so much

French can use both autant and beaucoup, but they are not always exactly the same.

• beaucoup often simply means a lot
• autant often emphasizes that much / so much, sometimes with a sense of comparison or surprise

Here, pourquoi ... sourit autant sounds natural for why is he smiling so much?

Because this is an indirect question, not a direct one.

In a direct question, French often uses inversion:

• Pourquoi le fiancé de Marie sourit-il autant ?

But after a verb like demande, French usually uses normal statement word order:

• Mon neveu demande pourquoi le fiancé de Marie sourit autant.

That is very common in French:

• Je sais où il habite.
• Elle demande quand tu pars.

So the sentence structure is completely normal.

Because la bague refers to a specific ring that both speaker and listener can identify from context.

Since the sentence mentions the fiancé of Marie, the ring is naturally understood as the engagement ring. So French uses the definite article:

• la bague = the ring

If you said une bague, it would sound more like a ring, any ring, not the particular one everyone is thinking about.

Because after vouloir (to want), French normally uses an infinitive.

• veut = wants
• voir = to see

So:

• veut voir = wants to see

This is the same pattern as in English:

• wants to see
• wants to eat
• wants to go

French examples:

• Elle veut partir. = She wants to leave.
• Nous voulons comprendre. = We want to understand.

By itself, bague just means ring.

But in this sentence, the context strongly suggests an engagement ring, because it mentions Marie’s fiancé. So la bague is very likely understood as the ring associated with the engagement.

French does not need to say bague de fiançailles every time if the context already makes it obvious.

In theory, demander can sometimes mean to request or to ask for, depending on the structure.

But here, because it is followed by pourquoi..., it clearly means asks:

• Mon neveu demande pourquoi... = My nephew asks why...

If it meant asks for something, the structure would be different, for example:

• Il demande de l’aide. = He asks for help.

So in this sentence, demande definitely means asks.