Mes parents s'inquiètent quand je ne réponds pas à leurs messages.

The s' is a reflexive (or “pronominal”) pronoun that is part of the verb s'inquiéter.

• 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.”

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.

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.

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.

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.

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.

• 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.

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.

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.

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

1. Je ne réponds pas à leurs messages.

   • Focuses on the messages (things) you’re not answering.

2. 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.