Learn Hardcore French: Paul est honnête, mais sa sœur est parfois paresseuse. - Paul is honest, but his sister is sometimes lazy.

Because French possessive adjectives agree with the gender and number of the thing possessed, not with the possessor. Sœur is feminine singular, so you use sa. Use son only when the possessed noun is masculine singular, or when a feminine noun begins with a vowel sound or mute h (to avoid a hiatus), e.g., son amie, son histoire. Since sœur starts with an s sound, it stays sa sœur.

Why is it “paresseuse” and not “paresseux”?

The adjective must agree with the noun. Paresseux is the masculine form; the feminine form is paresseuse.

Masculine singular: paresseux (son frère est paresseux)
Feminine singular: paresseuse (sa sœur est paresseuse)
Masculine plural: paresseux
Feminine plural: paresseuses

Can I move “parfois” to another place in the sentence?

Yes. Common, natural options:

Paul est honnête, mais sa sœur est parfois paresseuse. (most neutral)
Parfois, sa sœur est paresseuse. (fronted for emphasis) You may also hear Sa sœur est paresseuse parfois, which is possible but sounds more conversational or afterthought-like. Synonyms: quelquefois (a bit more formal), de temps en temps (neutral), des fois (informal/regional).

Is the comma before “mais” required?

It’s standard (and recommended) to put a comma before mais when linking two independent clauses with their own subjects, as here. You’ll also see it without a comma in short sentences, but the comma is good style.

How do you pronounce the whole sentence?

Approximate IPA: (pɔl ɛ t‿ɔnɛt | mɛ sa sœʁ ɛ paʁfwa paʁesøz)

est honnête: often with liaison: [ɛ t‿ɔnɛt]
sœur: [sœʁ] (like an “uh” sound but rounded)
parfois: [paʁfwa] (final s is silent)
paresseuse: [paʁesøz] (the -euse is a “-uhz” with rounded lips) Tip: The h in honnête is silent; the œ in sœur is the open “eu” sound; the “eu” in -euse is the closed “eu.”

What’s going on with the accents and the œ in “honnête” and “sœur”?

honnête must have the circumflex on ê. It’s not optional here and it affects spelling and pronunciation: honnête, honnêteté.
sœur uses the ligature œ. It’s best to write it; if you can’t type it, soeur is commonly accepted in plain text, but the ligature is the standard form.

Why does “honnête” have two n’s?

The double n keeps the “o” from becoming nasal. Compare:

bon [bɔ̃] vs bonne [bɔn] Likewise, honnête is pronounced [ɔn-], not with a nasalized “on.” Historically, the spelling also reflects old forms related to Latin “honestus.”

Is there liaison between “est” and “honnête”?

Yes. Because honnête begins with a vowel sound (the h is mute), you commonly make a t-liaison: est honnête → [ɛ t‿ɔnɛt]. In careful speech it sounds natural to include it.

Could I say “C’est honnête” instead of “Paul est honnête”?

Not for describing Paul. With a named or clear subject, use Paul est honnête or Il est honnête. C’est honnête is used for evaluating an action or situation: C’est honnête de ta part (“That’s honest of you”).

How would this change for plurals or different relatives?

Plural sisters: Ses sœurs sont parfois paresseuses.
A brother: Son frère est parfois paresseux.
Paul and his sister: Paul et sa sœur sont parfois paresseux. (mixed gender → masculine plural)
Two women: Pauline et sa sœur sont parfois paresseuses.

Is calling someone “paresseuse” rude?

It can be blunt or negative. To soften it, add a mitigator:

un peu/parfois/plutôt paresseuse
Or rephrase: elle manque parfois d’énergie / de motivation Stronger or more colloquial synonyms include fainéante (or fainéant), flemmarde (slang).

Can I replace “mais” with something like “cependant,” “pourtant,” or “alors que”?

Yes, with small structural changes:

Paul est honnête; cependant/pourtant, sa sœur est parfois paresseuse. (conjunctive adverbs; need punctuation)
Paul est honnête, alors que sa sœur est parfois paresseuse. (subordinating conjunction within one sentence)
Cependant and pourtant feel a bit more formal than mais.