Learn Hardcore French: Mes parents s'inquiètent quand je ne réponds pas à leurs messages. - My parents worry when I do not answer their messages.

inquiéter = to worry (someone)
- e.g. Cette situation m'inquiète. = This situation worries me.
s'inquiéter = to worry / to be worried (oneself)
- e.g. Mes parents s'inquiètent. = My parents are worried / get worried.

So you can’t just drop the s'; it’s part of the verb form meaning “to worry (oneself)” or “to get worried.”

Why does s'inquiètent end in -ent?

Because the subject is mes parents, which is third person plural (ils/elles form).

Subject: mes parents → ils
Verb: s'inquiéter in the present tense
- ils s'inquiètent

The -ent at the end is the normal spelling for ils/elles in the present tense.
Note: The -ent is silent in pronunciation: ils s'inquiètent is pronounced like il s'inquiète.

What is the difference between s'inquiéter and inquiéter?

They are related but used differently:

inquiéter quelqu'un = to worry someone / to make someone anxious
- Cette nouvelle inquiète mes parents.
  This news worries my parents.
s'inquiéter (pour / de) = to worry / to be worried (about)
- Mes parents s'inquiètent pour moi.
  My parents are worried about me.

In your sentence, we want “my parents are worried when…”, so we use s'inquiètent, not just inquiètent.

Could I instead say Mes parents sont inquiets quand je ne réponds pas…?

Yes, and it would be correct, with a small change:

Mes parents sont inquiets quand je ne réponds pas à leurs messages.

This uses:

être inquiet / inquiets = to be worried

Nuance:

Mes parents s'inquiètent… → focuses more on the process or reaction (they get worried each time).
Mes parents sont inquiets… → describes their state (they are in a worried state when…).

Both are natural; your original sentence is slightly more dynamic.

Why is quand followed by a present tense (je ne réponds pas) instead of a future tense like in English?

In French, after quand (when) to talk about general, repeated situations or future events, you normally use the present tense, not the future:

Quand je ne réponds pas, mes parents s'inquiètent.
Literally: When I do not answer, my parents worry.

English often uses the present + present here too, but with the future it stays different:

French future: Quand je ne répondrai pas, ils s'inquiéteront. (possible but more specific, one future event)
English: “When I don’t answer, they worry.” / “When I don’t answer, they will worry.”

For general habits, French sticks with the present on both verbs.

How does the negation ne … pas work in je ne réponds pas?

French negation usually “wraps around” the conjugated verb:

je réponds → je ne réponds pas = I don’t answer / I’m not answering

Structure:

ne (or n' before a vowel) goes before the verb
pas goes after the verb

In spoken informal French, the ne is often dropped:

Written: Je ne réponds pas.
Very common in speech: Je réponds pas.

But in standard writing (like your sentence), you keep ne … pas.

Why is it réponds and not réponse?

réponds is a verb form (1st person singular of répondre in the present).
réponse is a noun (an answer).

Conjugation of répondre in the present:

je réponds
tu réponds
il/elle répond
nous répondons
vous répondez
ils/elles répondent

So in “when I do not answer”, you need the verb je réponds, not the noun réponse.

Why do we say répondre à something, so à leurs messages, and not just répondre leurs messages?

The verb répondre in French is used with the preposition à:

répondre à quelqu'un = to answer someone
répondre à quelque chose = to answer something

So:

Je réponds à leurs messages. = I answer their messages.
You cannot say je réponds leurs messages in standard French; it sounds wrong.

Think of répondre à as a fixed combination: the à is required.

Why is it leurs messages and not leur messages?

Leur / leurs here is a possessive adjective (their):

leur (no -s) = with a singular noun
- leur message = their message
leurs (with -s) = with a plural noun
- leurs messages = their messages

The form of leur/leurs agrees with the number of the noun possessed, not with the owners:

one owner, several owners → still leur or leurs depending on message/messages:
- Il lit leur message. (one message, from several people)
- Il lit leurs messages. (several messages, from one or several people)

In your sentence, messages is plural, so you need leurs.

Can I replace à leurs messages with a pronoun, like je ne leur réponds pas? How does that work?

Yes, you can, but it slightly changes what you’re emphasizing.

Je ne réponds pas à leurs messages.
- Focuses on the messages (things) you’re not answering.
Je ne leur réponds pas.
- Uses leur as an indirect object pronoun = to them.
- This focuses more on the people you’re not answering.

If you use the pronoun, the word order with negation is:

Je ne leur réponds pas.
- ne
  - pronoun
    - verb
      - pas

Both are grammatically correct in French; the original just spells out “their messages” explicitly.