Vrijeme brzo prolazi kad učimo zajedno.

A word-by-word breakdown looks like this:

• vrijeme = time
• brzo = quickly / fast
• prolazi = passes / goes by
• kad = when
• učimo = we learn / we are learning
• zajedno = together

So the structure is very close to English:

• Vrijeme brzo prolazi = Time passes quickly
• kad učimo zajedno = when we learn together

Yes, vrijeme can mean both time and weather, depending on context.

In this sentence, vrijeme brzo prolazi clearly means time passes quickly, so vrijeme means time.

Examples:

• Vrijeme je lijepo. = The weather is nice.
• Nemam vremena. = I don’t have time.

So learners need to rely on context to know which meaning is intended.

Prolazi is the 3rd person singular present tense of prolaziti (to pass, to go by).

It is 3rd person singular because the subject is vrijeme, which is grammatically singular:

• vrijeme prolazi = time passes

A few matching examples:

• Dan brzo prolazi. = The day passes quickly.
• Život prolazi. = Life passes.

So prolazi agrees with vrijeme.

Because brzo is an adverb here, not an adjective.

• brz / brza / brzo can be adjective forms meaning fast
• but brzo can also function as an adverb meaning quickly

In this sentence, it describes how time passes:

• Vrijeme prolazi kako? → brzo
• How does time pass? → quickly

Compare:

• brz vlak = a fast train → adjective
• vlak ide brzo = the train goes fast/quickly → adverb

So here brzo is exactly what you want.

Both kad and kada mean when.

• kad is shorter and very common in everyday speech
• kada is a bit more formal or emphatic, but also completely normal

So these are both possible:

• Vrijeme brzo prolazi kad učimo zajedno.
• Vrijeme brzo prolazi kada učimo zajedno.

The meaning does not really change here.

Učimo is the 1st person plural present tense of učiti (to learn, sometimes to study depending on context).

So:

• učim = I learn / I am learning
• učiš = you learn
• uči = he/she/it learns
• učimo = we learn
• učite = you all learn
• uče = they learn

In this sentence, učimo means we learn or we are learning.

Croatian present tense often covers both English simple present and present continuous, so učimo can mean either depending on context.

In Croatian, subject pronouns are often omitted because the verb ending already shows the person and number.

Here, učimo already tells you it means we learn, so mi is unnecessary.

• učimo = we learn
• mi učimo = we learn, but with extra emphasis on we

So the sentence does not need mi.

You could say:

• Vrijeme brzo prolazi kad mi učimo zajedno.

But this would usually sound more emphatic, like when we learn together (not someone else).

Because Croatian word order is flexible, but this is a very natural placement.

Here zajedno modifies učimo:

• učimo zajedno = we learn together

Putting zajedno at the end sounds smooth and neutral.

You may also hear variations like:

• Kad zajedno učimo, vrijeme brzo prolazi.
• Vrijeme brzo prolazi kad zajedno učimo.

These are also correct. The version you were given is simple and natural.

It can potentially suggest either, because učiti is a broad verb.

Depending on context, učiti can mean:

• to learn
• to study

In this sentence, the idea is probably when we learn together, but in real life it could also be understood as when we study together.

For example:

• Učimo hrvatski. = We are learning Croatian.
• Učim za ispit. = I’m studying for an exam.

So the exact nuance comes from context.

The sentence is in the present tense:

• prolazi = passes / is passing
• učimo = we learn / we are learning

Croatian often uses the present tense for:

• general truths
• repeated situations
• things happening now

This sentence expresses a general idea:

• Time passes quickly when we learn together.

It is not tied to just one single moment; it sounds like a general observation.

Yes, prolaziti is a very common and natural verb for time passing or going by.

Examples:

• Vrijeme prolazi. = Time passes.
• Dani prolaze brzo. = Days pass quickly.

Another common expression is:

• Vrijeme leti. = Time flies.

So prolazi is perfectly normal and slightly more neutral than leti.

A simple pronunciation guide:

• Vrijeme ≈ VRYE-meh
• brzo ≈ BER-zoh or BR-zoh
• prolazi ≈ proh-LAH-zee
• kad ≈ kahd
• učimo ≈ OO-chee-moh
• zajedno ≈ zah-YED-noh

A few useful sound notes:

• j is pronounced like English y
• č is like ch in church
• ž is like the s in measure
• lj, nj, etc. are special Croatian sounds, though they do not appear here

A natural rhythm would be:

VRI-je-me BR-zo pro-LA-zi kad U-či-mo za-JED-no

Yes, absolutely.

These two versions mean the same thing:

• Vrijeme brzo prolazi kad učimo zajedno.
• Kad učimo zajedno, vrijeme brzo prolazi.

The difference is mostly about focus and style:

• starting with Vrijeme brzo prolazi... puts the emphasis first on time passing quickly
• starting with Kad učimo zajedno... puts the situation first: when we learn together

Both are natural Croatian.