Quand le film commence, on s’assoit sur le canapé.

Quand means when.

In this sentence, it introduces a time clause:

• Quand le film commence = When the movie starts
• on s’assoit sur le canapé = we sit down on the couch

Unlike English, French usually keeps a normal subject-verb order after quand:

• Quand le film commence...
• literally: When the movie starts...

So you do not need to invert anything here.

This is a very common question.

In Quand le film commence, on s’assoit sur le canapé, the present tense suggests a habitual or general action:

• Whenever the movie starts, we sit down on the couch.
• When the movie starts, we sit down on the couch (as a routine)

So this sentence sounds like something that regularly happens.

If you were talking about one specific future movie, French would usually use the future in both parts:

• Quand le film commencera, on s’assiéra sur le canapé.

That is an important difference from English, which often uses the present after when for future meaning.

On is extremely common in spoken and everyday French.

Here, on most likely means we:

• on s’assoit = we sit down

Even though on originally means something like one/people, it is very often used instead of nous in modern French conversation.

So:

• Nous nous asseyons sur le canapé = more formal or careful
• On s’assoit sur le canapé = more natural in everyday speech

Because the verb is s’asseoir, which is a reflexive verb.

• asseoir = to seat, to place someone in a sitting position
• s’asseoir = to sit down

So on s’assoit literally includes the reflexive pronoun:

• on = we
• s’ = ourselves / oneself
• assoit = sits

In natural English, we do not translate the reflexive part word for word. We simply say:

• we sit down

This is because of elision.

The reflexive pronoun is normally se, but before a vowel sound it becomes s’:

• se coucher
• se laver
• but s’asseoir
• s’appeler

Since asseoir begins with a vowel sound, se changes to s’:

• on s’assoit

Yes. S’assoit is a standard present-tense form of s’asseoir.

French learners often notice that s’asseoir can have more than one accepted conjugation pattern. For example, you may also see forms based on assey- in some contexts.

But for this sentence, on s’assoit is completely normal and natural.

So you can safely learn:

• je m’assois
• tu t’assois
• il/elle/on s’assoit

Here it most naturally means we sit down.

That is because s’asseoir usually describes the action of moving into a seated position.

So:

• on s’assoit = we sit down

If you wanted to emphasize being in a seated position rather than the action of sitting down, French might use other wording depending on context.

Because in French, as in English, you normally sit on a couch/sofa, not in it.

So:

• sur le canapé = on the couch

This is the normal preposition to use here.

French usually needs an article where English sometimes does too, and sometimes does not.

Here, le canapé means the couch.

So:

• sur le canapé = on the couch

In context, this usually refers to the couch that is known in the situation, such as the one in the living room.

Yes, absolutely.

That sentence is grammatically correct and means the same thing:

• Nous nous asseyons sur le canapé.

However, it sounds more formal or written. In everyday spoken French, people very often prefer:

• On s’assoit sur le canapé.

So both are correct, but on is usually more natural in conversation.

A simple pronunciation guide would be:

Quand le film commence, on s’assoit sur le canapé.
kahn luh feelm koh-mahns, on sa-swah syr luh ka-na-pay

A few helpful notes:

• quand sounds roughly like kahn
• film is pronounced clearly, with the l heard
• commence has a nasal vowel in the second syllable
• s’assoit sounds like sa-swah
• canapé ends with an ay sound

Most naturally, it describes a general habit or repeated situation.

So it means something like:

• Whenever the movie starts, we sit down on the couch.

That is because both verbs are in the present tense:

• commence
• s’assoit

If you wanted a single future event, French would more likely say:

• Quand le film commencera, on s’assiéra sur le canapé.