Learn Hardcore Croatian: Vrijeme se brzo mijenja u gradu. - The weather changes quickly in the city.

You tell from context and from what normally “changes quickly” in this kind of sentence. Time passes, but we usually say that weather changes. If you wanted to emphasize “time” (as in life, eras, etc.), you’d normally make the context clearer, for example:

Vrijeme se mijenja, ljudi se mijenjaju. – Times change, people change.

Why is there no word for the in the sentence?

Croatian has no articles like English the or a/an. The noun itself, plus context and case endings, carry the meaning.

So:

Vrijeme se brzo mijenja u gradu.
can mean:
- The weather changes quickly in the city.
- Weather changes quickly in the city.

English must choose an article; Croatian doesn’t use any.

What does se mean in se mijenja? Is it necessary?

Se is a reflexive clitic. In this sentence it makes the verb mijenjati (“to change”) into its reflexive / intransitive form mijenjati se (“to change, to change itself”).

mijenjati (nešto) – to change something (transitive)
- Mijenjam posao. – I’m changing (my) job.
mijenjati se – to change (by itself), to be changing (intransitive)
- Vrijeme se mijenja. – The weather is changing.

Without se, the verb mijenjati expects a direct object.
Vrijeme brzo mijenja u gradu sounds incomplete and ungrammatical, like “The weather quickly changes … (what?) in the city.”

So in this meaning (“weather changes”), se is necessary.

Can I put se after brzo, like Vrijeme brzo se mijenja u gradu?

Normally, no. Se is a clitic and must stand in a specific position in the clause, usually right after the first stressed word (often the subject).

Grammatical and natural:

Vrijeme se brzo mijenja u gradu.
U gradu se vrijeme brzo mijenja.

Odd or wrong in standard Croatian:

Vrijeme brzo se mijenja u gradu. ✗
(this breaks the usual clitic placement rule)

So you should keep se right after the first stressed element of the clause (here, vrijeme or u gradu).

What’s the difference between mijenjati and mijenjati se?

mijenjati = to change something (transitive)
- Mijenjam stan. – I’m changing (my) apartment.
- On mijenja posao. – He is changing (his) job.
mijenjati se = to change (by itself), to be changing (reflexive / intransitive)
- Vrijeme se mijenja. – The weather is changing.
- Svijet se mijenja. – The world is changing.

In your sentence, vrijeme is not changing some other object; it is what is changing, so you must use mijenjati se.

Why is it brzo and not brz?

Brz is an adjective = “fast, quick” (describes a noun).
Brzo is the adverb = “quickly, fast” (describes a verb).

brz vlak – a fast train (adjective)
Vlak ide brzo. – The train goes quickly. (adverb)

In your sentence, brzo describes how the weather changes:

Vrijeme se brzo mijenja. – The weather changes quickly.

So you need the adverb brzo, not the adjective brz.

Can I move brzo to a different place, like Vrijeme se mijenja brzo u gradu?

Yes, Croatian word order is relatively flexible. These are all grammatically possible:

Vrijeme se brzo mijenja u gradu.
Vrijeme se mijenja brzo u gradu.

The difference is mostly about rhythm and emphasis, not grammar.
The most neutral, natural order is usually:

Vrijeme se brzo mijenja u gradu.

Putting brzo right before the verb is very common when you’re describing the manner of the action.

Why is it u gradu and not u grad?

Because u here means “in” (location), and it requires the locative case:

u + locative → “in (somewhere)”

Grad (city) in locative singular is gradu:

nominative: grad – city
locative: u gradu – in the city

If you said u grad (accusative), it would normally mean “to the city” (movement towards):

Idem u grad. – I’m going to the city.

So:

Vrijeme se brzo mijenja u gradu. – The weather changes quickly in the city (location).
Idem u grad. – I’m going to the city (direction / movement).

What case is gradu, and what is the usual pattern for this noun?

Gradu is locative singular of grad (“city”).

For grad (masculine, hard consonant), the main singular forms are:

nominative: grad – a/the city
genitive: grada – of a/the city
dative: gradu – to/for the city
accusative: grad – (to) the city, the city (object)
locative: u gradu, o gradu – in/about the city
instrumental: gradom – with/by the city

Locative and dative singular have the same form (gradu) for many masculine nouns; you tell them apart from the preposition and meaning.

Here, u gradu clearly signals locative (“in the city”).

Can I start the sentence with U gradu? For example: U gradu se vrijeme brzo mijenja.

Yes. That is completely grammatical and very natural:

U gradu se vrijeme brzo mijenja.

Compared to:

Vrijeme se brzo mijenja u gradu.

Both mean “The weather changes quickly in the city.”
Starting with U gradu puts slightly more emphasis on the place (“In the city, the weather changes quickly”), but the basic meaning is the same.

What gender is vrijeme, and does that affect the verb mijenja?

Vrijeme is neuter singular.

For present-tense verbs, the ending (here -a) in mijenja tells you it is 3rd person singular, which matches vrijeme as the subject, regardless of gender.

On se mijenja. – He is changing. (masculine, 3rd sg)
Ona se mijenja. – She is changing. (feminine, 3rd sg)
Vrijeme se mijenja. – The weather is changing. (neuter, 3rd sg)

So the main thing is number and person; the verb doesn’t change its form based on gender in the present tense.

Can this sentence also mean “Time changes quickly in the city” (not weather)?

Yes, in theory, because vrijeme can mean time.
However, without extra context, most native speakers will first understand it as weather.

If you really want to stress the “time” meaning (as in life, social conditions, etc.), you’d usually add context:

U gradu se vremena brzo mijenjaju. – Times change quickly in the city.
(Using plural vremena = “times” makes the meaning clearer.)
U gradu se vrijeme brzo mijenja – ljudi, moda, običaji.
– Time changes quickly in the city – people, fashion, customs.

Your original sentence, by itself, is most naturally understood as about the weather.