Learn Hardcore French: Ils répondent dans la messagerie après le dîner. - They reply in the messaging app after dinner.

a group of only males, or
a mixed‑gender group (males + females), or
sometimes a group of things that are grammatically masculine plural (e.g. les messages → ils).

If the group is only female (or only feminine nouns), you’d normally use elles instead. So:

Ils répondent… = They (masc./mixed group or masculine things) answer…
Elles répondent… = They (all women or feminine things) answer…

What tense is répondent, and how is répondre conjugated in the present?

Répondent is the present tense, third person plural of the verb répondre (to answer / to reply).

Répondre in the present tense:

je réponds – I answer
tu réponds – you answer (singular, informal)
il / elle / on répond – he / she / one answers
nous répondons – we answer
vous répondez – you answer (plural or formal)
ils / elles répondent – they answer

In French, the present can often correspond to English:

They answer
They are answering
They do answer

All of these can be ils répondent, depending on context.

Is répondre used the same way as “to answer” in English? Don’t we usually say répondre à something?

Yes, répondre often takes à when you say what or whom you are answering:

répondre à quelqu’un – to answer someone
répondre à un message / à une question – to answer a message / a question

In your sentence, dans la messagerie tells you where they are replying (in the messaging system), not what they are replying to. The object is understood from context (e.g. messages, emails, etc.).

Examples:

Ils répondent aux messages. – They answer the messages.
Ils répondent dans la messagerie. – They reply in the messaging app/system.

So répondre à is about the target of the answer; dans / par / sur etc. can describe the place or channel.

Why is it dans la messagerie and not just dans messagerie? Do I always need la?

In French, common nouns almost always need an article (le, la, les, un, une, des) unless there’s a specific grammatical reason to omit it.

la messagerie = the messaging system/mailbox
dans la messagerie = in the messaging system

Without an article (dans messagerie), it sounds incomplete or like a label rather than a normal sentence.

You might see the bare noun in:

titles/labels: Messagerie (button on a website)
technical shorthand or notes: Problème messagerie

But in a full normal sentence, dans la messagerie is standard.

What exactly does la messagerie mean here? Is it email, voicemail, or texting?

La messagerie is a general word for a messaging or message-storage system. Its exact meaning depends on context:

la messagerie électronique – email system
la messagerie vocale – voicemail
in many modern contexts, la messagerie alone can mean:
- the messaging section of a website/app (DMs),
- the inbox, or
- a chat / messaging app.

In your sentence, with no extra context, dans la messagerie most naturally suggests “in the messaging app / inbox / DM system”. If you need to be precise, you’d usually add a qualifier:

dans la messagerie vocale – in the voicemail
dans la messagerie de l’application – in the app’s messaging

Why is the preposition dans used? Could I say sur la messagerie or par la messagerie instead?

Each preposition slightly changes the nuance:

dans la messagerie – in the messaging system (inside that interface/inbox).
sur la messagerie – literally on the messaging system; could be heard, but is less standard and often sounds more like talking about a platform (like a website) than an inbox.
par la messagerie – via the messaging system, focusing on it as a means or channel of communication.

So:

Ils répondent dans la messagerie.
They are answering inside the messaging interface/inbox.
Ils répondent par la messagerie.
They answer via the messaging system (not by phone, not in person, etc.).

In everyday speech, dans la messagerie (place) and par la messagerie (means) are the two most natural choices; dans is very common if you imagine opening an inbox and answering there.

Why is it après le dîner and not just après dîner? Are both correct?

Both exist, but they differ slightly:

après le dîner
- Very common and neutral.
- Literally: after the dinner / after dinner.
- Uses the definite article le, because meals in French often take le / la when you refer to them in a general or habitual way:
  - le petit-déjeuner, le déjeuner, le dîner.
après dîner
- Also correct, a bit more compact or somewhat literary/old-fashioned in feel.
- Feels more like after eating dinner / once dinner is done.

In everyday modern speech, après le dîner is probably what you’ll hear most often.

What’s the word order rule here? Could I move après le dîner to another place in the sentence?

Yes, you can move it. All of these are grammatically correct:

Ils répondent dans la messagerie après le dîner.
Après le dîner, ils répondent dans la messagerie.
Ils, après le dîner, répondent dans la messagerie. (possible but feels heavier / more marked)

General guideline in French:

Core sentence: subject + verb + (direct/indirect objects).
Place complements (where?) often follow that.
Time complements (when?) can go at the beginning or end, depending on emphasis.

Putting Après le dîner at the start emphasizes the time more strongly:

Après le dîner, ils répondent dans la messagerie.
Focus: it’s specifically after dinner that they do this.

How do you pronounce ils répondent dans la messagerie? Which letters are silent?

Approximate pronunciation (in IPA):
[il ʁepɔ̃ dɑ̃ la mesɑʒʁi]

Key points:

ils → [il]
- s is silent here; no liaison before répondent because it starts with r (a consonant).
répondent → [ʁepɔ̃]
- Final -ent is silent in ils répondent.
- Same sound as je réponds or il répond in the present; you rely on context for the subject.
dans → [dɑ̃]
- Final s is silent.
la → [la]
messagerie → [mesɑʒʁi]
- -ie at the end is like [i].
- ssa → [sa]; the e at the start is [ə] or [e] depending on accent, often [me-].
- g
  - e → [ʒ], like the s in measure.

Spoken smoothly: [il ʁepɔ̃ dɑ̃ la mesɑʒʁi].

Is this sentence formal or informal? Could I use it in writing (emails, texts, etc.)?

The sentence is neutral in register:

Vocabulary is standard: répondre, messagerie, dîner.
There’s no slang or strong formality.

You can use this kind of sentence:

in everyday conversation,
in emails or messages to colleagues,
in neutral written texts describing habits or procedures.

To make it feel a bit more formal or explicit, you might add details:

Ils répondent dans la messagerie après le dîner chaque soir.
Ils répondent aux clients dans la messagerie après le dîner.

But as it stands, it’s perfectly acceptable in both speech and writing.