Le temps passe vite quand nous parlons français ensemble.

In French, nouns usually need an article, even when English would often leave it out.

So le temps literally means the time, but in French it is also the normal way to say time in a general sense.

• Le temps passe vite. = Time passes quickly.

French often uses the definite article where English uses no article.

Because the verb must agree with the subject in French.

Here, the subject is le temps, which is third person singular, so the verb passer becomes:

• je passe
• tu passes
• il / elle / on passe
• nous passons
• vous passez
• ils / elles passent

Since le temps is singular, we use passe.

Vite means quickly or fast. It is modifying the verb passe, so it tells us how time passes.

• Le temps passe vite. = Time passes quickly / fast.

You could also say rapidement, but vite is more common and natural in everyday speech.

• vite = very common, simple, conversational
• rapidement = also correct, slightly more formal or neutral

French often uses the present tense after quand when talking about something that happens generally or regularly.

Here the idea is:

• Time passes quickly when we speak French together.

This is a general truth or repeated situation, so the present tense is natural in both parts:

• Le temps passe vite
• quand nous parlons français ensemble

French does this just like English in sentences such as When we speak French together, time flies.

Both are possible, but they have different tones.

• nous parlons = more standard, careful, written, or formal
• on parle = very common in everyday spoken French

So:

• quand nous parlons français ensemble
• quand on parle français ensemble

Both mean when we speak French together.

Learners should know that native speakers often prefer on in conversation, but nous is still completely correct and important to learn.

Because parlons is the nous form of the verb parler in the present tense.

Present tense of parler:

• je parle
• tu parles
• il / elle / on parle
• nous parlons
• vous parlez
• ils / elles parlent

Since the subject is nous, the ending must be -ons.

Because here français means French as a language, and after verbs like parler, French usually does not use an article before the language.

• parler français = to speak French
• apprendre l’anglais can happen in some contexts, but after parler, normally just parler anglais / français / espagnol

So:

• nous parlons français = we speak French

Also, français is written with a lowercase f in French when it means the language.

The ç is called a cédille. It shows that the c should be pronounced like s, not like k.

Without the cedilla:

• francais would suggest a different pronunciation

With the cedilla:

• français is pronounced with an s sound

This spelling is standard and must be written correctly in French.

Ensemble means together.

It refers to nous parlons français:

• nous parlons français ensemble = we speak French together

Its position is natural at the end of the clause. French often places words like ensemble after the main verb phrase.

You could think of the clause as:

• quand = when
• nous parlons français = we speak French
• ensemble = together

A careful approximate pronunciation is:

luh tahn pass veet kahn noo parlon frahn-say ahn-sombl

A few useful points:

• temps is pronounced roughly like tahn; the final ps is silent
• passe sounds like pass
• vite sounds like veet
• quand sounds roughly like kahn
• nous sounds like noo
• parlons sounds like par-lon; the final s is silent
• français sounds roughly like frahn-say
• ensemble sounds roughly like ahn-sombl

French pronunciation is not always obvious from spelling, so this is a sentence worth listening to several times.

It most naturally expresses a general truth or a repeated experience:

• Time goes by quickly when we speak French together.

It suggests that whenever this happens, it feels like time passes fast.

If you wanted to emphasize one specific occasion, French would usually give more context, for example with a past tense:

• Le temps est passé vite quand nous avons parlé français ensemble.

But your original sentence is best understood as a general statement.

Yes. Le temps file is also natural and means something like time flies.

Compare:

• Le temps passe vite = time passes quickly
• Le temps file = time flies

Both are good French, but Le temps passe vite is very transparent for learners and very common. Le temps file is a bit more idiomatic.