Ce canapé sert de lit quand mes parents arrivent.

Because canapé is a masculine singular noun and it begins with a consonant.

French uses:

• ce before a masculine singular noun beginning with a consonant: ce canapé
• cet before a masculine singular noun beginning with a vowel or silent h: cet appartement
• cette before a feminine singular noun: cette chaise

So ce canapé is the correct form.

It can mean either one.

In everyday French, ce / cet / cette / ces often covers both this and that. The exact meaning depends on context.

If French speakers want to make the difference clearer, they can add:

• -ci for this
• -là for that

For example:

• ce canapé-ci = this sofa
• ce canapé-là = that sofa

In your sentence, plain ce canapé is completely normal.

It comes from the verb servir.

The pattern servir de + noun means to serve as, to function as, or to be used as.

So:

• sert = serves
• sert de lit = serves as a bed

This is a very common French structure:

• Cette boîte sert de table.
• Cette pièce sert de bureau.

It is not about serving someone; it is about a function or use.

After servir de, French usually uses de + noun without an article when you are talking about a role or function.

So:

• servir de lit = to serve as a bed
• servir de table = to serve as a table
• servir de bureau = to serve as an office/workspace

This is different from English, where you often need a:

• It serves as a bed

In French, the article is normally dropped in this structure.

No. Sert is a form of servir, not être.

Here is the present tense of servir:

• je sers
• tu sers
• il/elle/on sert
• nous servons
• vous servez
• ils/elles servent

So in your sentence, ce canapé sert de lit, the subject is singular, so French uses sert.

Because parents is plural, and French possessive adjectives agree with the thing possessed, not with the owner.

So:

• mon parent = my parent
• mes parents = my parents

Even though English uses my for both singular and plural, French changes the form:

• mon / ma / mes

Here, since parents is plural, you need mes.

Because the subject mes parents is plural.

The verb arriver in the present tense changes depending on the subject:

• j’arrive
• tu arrives
• il/elle/on arrive
• nous arrivons
• vous arrivez
• ils/elles arrivent

Since mes parents = they, French uses arrivent.

A useful pronunciation note: in most cases, the -ent ending of plural verbs in the present tense is not pronounced.

Because this sentence describes a habitual or regular situation.

The idea is:

• this sofa serves as a bed whenever my parents arrive

French often uses the present tense for things that happen regularly or generally.

If you were talking about one specific future occasion, French could use the future:

• Ce canapé servira de lit quand mes parents arriveront demain.

So in your sentence, the present tense suggests a repeated situation, not just one future event.

Literally, quand means when, but in this sentence it naturally has the sense of whenever because the whole statement is general or habitual.

So the idea is:

• when my parents arrive
• or more naturally in English: whenever my parents arrive

That is a very common use of quand in French.

Not with the same meaning.

• Ce canapé est un lit would mean the sofa is a bed.
• Ce canapé sert de lit means the sofa functions as a bed or is used as a bed.

So sert de lit is better if it is still really a sofa, but people use it like a bed when needed.

If the furniture is actually a sofa bed, French often says:

• C’est un canapé-lit.

Yes, it is natural and correct.

It sounds like a normal way to say that the sofa is used as a bed when the parents come to stay.

Depending on context, a French speaker might also say something slightly different, such as:

• Ce canapé sert de lit quand mes parents viennent.
• Ce canapé sert de lit à mes parents quand ils viennent.

But your original sentence is perfectly good French.