Learn Hardcore French: Paul change de sujet quand la conversation devient trop sérieuse. - Paul changes the subject when the conversation becomes too serious.

In French, with the verb changer you usually use the structure changer de + noun when you mean “to switch to a different X” (to change cars, jobs, topics, etc.), not “to modify X itself.”

changer de sujet = to change the subject (switch to another topic)
changer de travail = to change jobs
changer de vêtements = to change clothes

Changer le sujet would sound more like “modify the subject (itself),” which isn’t how French normally expresses this idea in everyday speech. So the natural idiom is changer de sujet.

What is the difference between changer and se changer here? Could we say Paul se change de sujet?

No, you cannot say se changer de sujet.

changer de sujet = to change the subject (transitive pattern changer de + noun)
se changer = to get changed (clothes), as in Je vais me changer. (“I’m going to change my clothes / get changed.”)

The reflexive form se changer de is not used with sujet. The correct and idiomatic expression is only changer de sujet.

Why is it change and not changes like in English?

French verbs are written differently from English:

Subject: Paul → third person singular
Verb: changer → present tense third person singular = il/elle change

French does not add an -s in the spelling for the third person singular of regular -er verbs. So:

je change
tu changes
il/elle/on change
nous changeons
vous changez
ils/elles changent

In English you need changes; in French it stays change.

Why is it quand and not lorsque? Are they interchangeable?

In this sentence, quand and lorsque are almost interchangeable:

Quand la conversation devient trop sérieuse…
Lorsque la conversation devient trop sérieuse…

Both can mean when. Differences:

quand is more common and neutral in everyday speech.
lorsque can sound a bit more formal or literary, and is especially frequent in written French.

Here, quand is the most natural choice in normal spoken French, but lorsque would still be correct.

Why is the verb devient and not deviens or deviennent?

Devient is the third person singular form of devenir in the present tense:

je deviens
tu deviens
il/elle/on devient
nous devenons
vous devenez
ils/elles deviennent

The subject is la conversation, which is singular (one conversation). So the verb must be devient.

Deviens would go with je or tu, and deviennent would go with a plural subject like les conversations.

Why is it trop sérieuse and not trop sérieux?

The adjective has to agree in gender and number with the noun it describes:

Noun: la conversation → feminine singular
Adjective: base form sérieux
- masculine singular: sérieux
- feminine singular: sérieuse
- masculine plural: sérieux
- feminine plural: sérieuses

Since conversation is feminine singular, the adjective must be feminine singular: sérieuse.

If it were le sujet, you would say le sujet devient trop sérieux (masculine).

Why is sérieuse an adjective here and not an adverb like sérieusement?

French uses an adjective to describe a state of something (what it is like), and an adverb to describe how an action is done.

La conversation devient trop sérieuse.
→ The conversation becomes too serious.
You are talking about the nature/state of the conversation, so you use the adjective sérieuse.
La conversation continue trop sérieusement.
→ The conversation continues too seriously.
Here you are describing the manner of continuing, so sérieusement is an adverb modifying the verb.

In the original sentence, the focus is on what the conversation is (serious), not on how it is happening, so the adjective is correct.

Why is it la conversation and not une conversation?

Using la here signals that we are talking about a specific, typical situation, not just any random conversation. There are two common ways to understand la conversation in this kind of sentence:

A specific conversation currently happening:
Paul change de sujet quand la conversation devient trop sérieuse.
→ In that conversation, at the moment it becomes too serious, he changes the subject.
A general habit (more likely here):
la conversation is like “the conversation in general, in that kind of context.”
→ Whenever the conversation (he’s in) gets too serious, he changes the subject.

Using une would sound like you are suddenly introducing a new, unspecified conversation, and it would not fit the habitual meaning as naturally.

Could we say Quand la conversation devient trop sérieuse, Paul change de sujet instead? Is the word order important?

Yes, that version is perfectly correct:

Paul change de sujet quand la conversation devient trop sérieuse.
Quand la conversation devient trop sérieuse, Paul change de sujet.

Both have the same meaning. In French, you can place the quand-clause:

after the main clause, or
before the main clause (usually with a comma).

Changing the order here is mostly a matter of style and emphasis, not grammar.

Why is the tense the simple present and not some future form, since it seems to describe what will happen when something occurs?

French uses the present tense much more widely than English, especially to talk about general truths and habits.

Here the present has a habitual meaning:

Paul change de sujet quand la conversation devient trop sérieuse.
= Paul tends to change the subject whenever the conversation gets too serious.

English also uses the present for habits (he changes), so this matches fairly well. You would not normally use the future in French here, because the sentence is not about a single future event but about what usually happens.

Is la conversation really the subject of devient even though it comes after quand?

Yes. In the clause quand la conversation devient trop sérieuse:

quand introduces the time clause (like “when”).
la conversation is the grammatical subject.
devient is the verb.

Word order in that clause remains subject–verb–rest, just like in a simple sentence:
La conversation devient trop sérieuse.

Adding quand at the beginning only turns it into a subordinate clause; it doesn’t change who the subject is.