Learn Hardcore French: Paul a l'habitude de lire un livre le soir. - Paul is used to reading a book in the evening.

Because a is the verb avoir in the 3rd person singular (he/she/it has). The preposition à (to/at) would be incorrect here. Quick check: if you can replace it with has, you need a without an accent: Paul has the habit…

Why is it l'habitude and not la habitude?

French elides la to l' before a vowel or a silent-h (here, habitude has a silent-h, called h muet). Hence l'habitude. With the indefinite article you’d say une habitude.

What is the role of de before lire, and could we use à instead?

The expression is avoir l'habitude de + infinitive/noun. So you must use de before lire. You cannot use à in this structure. Examples:

Verb: Paul a l'habitude de lire.
Noun: Paul a l'habitude de la lecture le soir.

Can I say Paul est habitué à lire un livre le soir? What’s the difference?

Yes. Être habitué à + infinitive/noun is also correct. Nuance:

Avoir l'habitude de highlights the habit/custom itself.
Être habitué à highlights being accustomed/used to something. In many contexts they’re interchangeable.

What tense is a here, and how do I talk about past habits?

A is present indicative (a general, current habit). For past habits:

Ongoing/regular in the past: Paul avait l'habitude de lire un livre le soir.
Simple repeated action (without naming the “habit”): Paul lisait un livre le soir.
A completed, no-longer-true habit: Paul a eu l'habitude de lire un livre le soir.

Is there a simpler way to express the same idea?

Yes. Common alternatives:

Paul lit un livre le soir.
D'habitude, Paul lit un livre le soir.
More formal: Paul a pour habitude de lire un livre le soir.
Similar idea: Paul a tendance à lire un livre le soir.

Why un livre and not des livres?

un livre often means “a (single) book” as the evening’s reading, typically the book he is currently working through; it does not imply finishing a whole book each evening.
des livres is also possible (Paul a l'habitude de lire des livres le soir) and emphasizes the general activity of reading books (plural) in the evenings. Other natural options: son livre, quelques pages, un chapitre.

Can I drop the object and just say Paul a l'habitude de lire le soir?

Yes. …de lire le soir means “to read in the evenings” without specifying what he reads.

What exactly does le soir convey? Does it mean every evening?

Le soir means “in the evening(s)” as a general, habitual time. It suggests regularity but not necessarily every single evening. To make it explicit, use tous les soirs or chaque soir. For a specific evening, say ce soir. For just one unspecified evening, un soir.

What’s the difference between le soir, la soirée, and la nuit?

le soir: evening (time of day, roughly after late afternoon until night).
la soirée: the duration/event of the evening (often when something is happening), e.g., une soirée (an evening party).
la nuit: night (late night/overnight hours).
Example: Paul lit le soir, not typically la nuit (which would be quite late).

Can I move le soir to the front?

Yes, for emphasis or style:

Le soir, Paul a l'habitude de lire un livre. You can also say: Paul lit un livre le soir, d'habitude.

How do I negate or ask a question with this structure?

Negation: Paul n'a pas l'habitude de lire un livre le soir.
Yes/no question (inversion): Paul a-t-il l'habitude de lire un livre le soir ?
Yes/no question (est-ce que): Est-ce que Paul a l'habitude de lire un livre le soir ?

Any pronunciation tips for the whole sentence?

a l'habitude: the h in habitude is silent, so it flows as “a-l’habitude” (hear it like “al-abitude”).
de lire: the e in de is very reduced, like “də”.
un has a nasal vowel; livre ends with a pronounced “vr”; le soir sounds like “lə swar” (no “z” sound).
Say it smoothly: “Paul a l’habitude də lire un livre lə swar.”

Is avoir l'habitude de formal?

It’s neutral and common. A more formal variant is avoir pour habitude de: Paul a pour habitude de lire un livre le soir.