Learn Hardcore French: À la maison, la boîte aux lettres est pleine; mon oncle l’ouvre avec sa clé. - At home, the mailbox is full; my uncle opens it with his key.

À la maison literally “at the house,” and idiomatically “at home.” It’s slightly impersonal and can refer to the speaker’s home or a previously mentioned home.
Chez moi means “at my place” and explicitly points to the speaker’s home.
To say “at my uncle’s (place),” use chez mon oncle, not “à la maison de mon oncle.”

Why is it la boîte aux lettres and not la boîte de lettres?

Boîte aux lettres is the fixed expression for “mailbox/letterbox.” The preposition à often marks purpose or intended use: “a box for letters.” Aux = à + les because lettres is plural. Using de would mean “a box of letters” (a container that currently has letters in it), not the installed mailbox. Other purpose compounds:

machine à laver (washing machine)
sac à dos (backpack)
verre à vin (wine glass)
brosse à dents (toothbrush)

Why is it pleine and not plein?

Agreement. Pleine is feminine singular to match la boîte. With verbs like être, adjectives agree in gender and number:

Feminine singular: La boîte est pleine.
Masculine singular: Le sac est plein.
Plural: Les boîtes sont pleines.

What does the semicolon do here? Do I need a space before it in French?

The semicolon links two closely related independent clauses: the mailbox is full; the uncle opens it. You could also use a period or a comma + et. French typographic convention uses a (thin, non‑breaking) space before “;” and a normal space after: …est pleine ; mon oncle… (In plain typing, people sometimes omit the thin space.)

What does l’ in l’ouvre stand for?

It’s the direct object pronoun la (referring to la boîte aux lettres) elided to l’ before a vowel sound. So l’ouvre = “opens it.” Don’t use y here; y replaces places or “à + thing,” not a direct object noun.

Could I just say mon oncle ouvre la boîte aux lettres instead of mon oncle l’ouvre?

Yes. If the noun hasn’t been mentioned yet, repeating it is fine: Mon oncle ouvre la boîte aux lettres. Once it’s known, French favors a pronoun: Mon oncle l’ouvre. Remember that object pronouns go before the verb.

How is ouvre conjugated? Why is there no -s?

Present tense of ouvrir:

je ouvre (j’ouvre)
tu ouvres
il/elle/on ouvre
nous ouvrons
vous ouvrez
ils/elles ouvrent So both je and il/elle take ouvre (no -s). Here, the subject mon oncle (il) explains the “he opens” meaning.

Why sa clé and not son clé? And is clé/clef both OK?

The possessive agrees with the thing possessed, not with the owner. Clé is feminine, so sa clé (“his/her key”). With a masculine noun: son oncle.
You use son before a feminine noun starting with a vowel sound for euphony: son amie (not sa amie). But clé starts with a consonant, so sa clé.
Clé and clef are both correct; clé is more common today.

Do accents and the apostrophe matter here (À, boîte, clé, l’ouvre)?

Yes.

À (preposition “at”) is different from a (verb “has”). Capital letters keep accents.
boîte normally has a circumflex on the i: î. Spelling it without can be seen as a mistake in careful writing.
clé takes é; cle is nonstandard (but clef is fine).
The apostrophe in l’ouvre is mandatory because of elision before a vowel sound. The typographic shape of the apostrophe doesn’t change meaning.

Can y replace à la maison or la boîte aux lettres here?

y can replace a place introduced by à/chez/dans/sur… if you’re repeating it: e.g., J’y suis for “I’m there (at home).” This sentence doesn’t repeat the place, so leaving À la maison is normal.
y cannot replace a direct object like la boîte aux lettres. That’s what le/la/les (l’) are for.

Any quick pronunciation tips for this sentence?

À la maison: [a la mɛzɔ̃] (final -n not pronounced; nasal vowel in -on).
boîte: [bwat] (oi = [wa]; the final -e is mute but keeps the -t pronounced).
aux: [o].
lettres: [lɛtʁ] (final -s silent).
est pleine: [ɛ plɛn] (the -t of est is silent here).
mon oncle: [mɔ̃ nɔ̃kl] (both vowels nasal).
l’ouvre: [luvʁ] (the l’ links to the vowel).
avec: [avɛk]; clé: [kle]. No liaison in aux lettres (the next word starts with a consonant).

How do negation and questions work with this object pronoun?

Negation: Mon oncle ne l’ouvre pas avec sa clé. (ne before the pronoun; pas after the verb)
Yes/no question: Est-ce qu’il l’ouvre avec sa clé ?
Inversion: L’ouvre-t-il avec sa clé ? French typography uses a (thin) space before ? as well.

How would I say it in the past? Any agreement to watch?

Mon oncle l’a ouverte avec sa clé.

Past participle: ouvert (irregular), but it agrees with a preceding direct object (la) → ouverte (add -e).
Plural object: Il les a ouvertes.

Why avec and not par?

Avec introduces the instrument/tool used (“with his key”). Par is mainly “by” (agent in the passive) or indicates cause/means in other senses, not the physical tool in an active sentence.

Can I move À la maison to another position?

Yes. Two natural positions:

Fronted: À la maison, la boîte aux lettres est pleine… (as given)
At the end: La boîte aux lettres est pleine à la maison… You can also insert it mid-sentence with commas for emphasis, but fronting or placing it at the end is most common.