Learn Hardcore French: Ses parents sont sérieux quand ils parlent des devoirs. - Her parents are serious when they talk about homework.

Ses = “his/her/their” when one person owns several things. It agrees with what is owned (plural: parents), not with the owner.
Use leurs only when several people are the owners: “their (several people’s) parents.”
Son/sa are for a single owned item (masculine/feminine singular): son père, sa mère. With plural “parents,” you need ses.

Why is the adjective sérieux used (with -x), and how does agreement work?

Sérieux is the masculine form; it’s the same in singular and plural (masc. sg = masc. pl = sérieux).
Parents is a masculine plural noun, so the adjective must be masculine plural: sont sérieux.
Feminine forms: sérieuse (sg), sérieuses (pl). You’d say ses mères sont sérieuses if you used the feminine noun mères.

Does quand here mean “when” or “whenever”? What about tenses?

With the present tense, quand usually has a habitual sense: “when/whenever they talk about homework.”
For future time in French, use the future in both clauses: Ils seront sérieux quand ils parleront des devoirs (not the English-style present after “when”).
To emphasize “whenever,” you can also use chaque fois que.

Why is the pronoun ils used? Could it be elles or eux?

Ils is the standard subject pronoun for a group that is mixed or not specified as all-female.
Elles is possible only if you clearly mean an all-female group (e.g., two mothers) and that is known in context.
Eux is a stressed pronoun; it cannot be the subject of a verb here.

Why parler de? What’s the difference between parler de and parler à?

Parler de [quelque chose] = to talk about something: parler des devoirs.
Parler à [quelqu’un] = to talk to someone: parler aux parents.
When you later replace “de + thing” with a pronoun, use en: Ils en parlent (“They talk about it”).

In des devoirs, is des the plural article “some,” or is it de + les? How should I interpret it?

Here, after parler de, des is often understood as the contraction of de + les (“about the homework” that’s known in context, e.g., the child’s homework).
If you want to speak about homework in general as a concept, you can say parler de devoirs (no article after de).

Can I use the singular devoir for “homework”?

In the school sense, French typically uses the plural: les devoirs = “homework.”
Un devoir = a single assignment/paper (countable task).
Le devoir can also mean “duty,” which is a different meaning.

Could I use lorsque instead of quand?

Yes: lorsque and quand are interchangeable in most time clauses. Lorsque is a bit more formal/literary.
You can’t use lorsque in direct questions (only quand ? works there).

Can I put the quand-clause first? Any punctuation changes?

Yes: Quand ils parlent des devoirs, ses parents sont sérieux.
When the time clause comes first, add a comma after it. No comma is needed when it comes last.

How should I pronounce key parts of the sentence?

ses: [se] (like “say” without the y-glide)
parents: [pa-ʁɑ̃]; final -s is silent
sont: [sɔ̃] (nasal vowel)
sérieux: [se-ʁjø]; final -x is silent; keep the accented é
quand: [kɑ̃]; nasal vowel; optional liaison before a vowel: quand ils can be [kɑ̃‿t‿il] (the -d links as a [t])
parlent: [paʁl]; final -ent is silent
devoirs: [də-vwaʁ]
No liaison between a noun and the following verb, so no link in parents sont.

Are sérieux, stricts, and sévères interchangeable?

Could I replace des devoirs with a pronoun?

Yes, use en for “about it/them”: Ses parents sont sérieux quand ils en parlent.
Keep word order: subject + verb + en (before the verb it belongs to).