Rozkvas vznikne z kvásku.

They are related, but not the same thing.

• kvásek = the starter or a small sourdough culture
• rozkvas = a larger ferment / build made from that starter

So the sentence says that the rozkvas is made or formed from the kvásek. In baking language, this is a very natural distinction.

Because the preposition z normally requires the genitive case.

Here:

• basic dictionary form: kvásek
• after z: kvásku

So:

• z kvásku = from the starter

This use of z often expresses origin, source, or material: something comes from something else.

Kvásek is a masculine inanimate noun, and in the genitive singular it changes to kvásku.

A learner does not have to memorize every detail immediately, but the important pattern is:

• nominative singular: kvásek
• genitive singular: kvásku

This kind of change is common with masculine nouns ending in -ek.

Vznikne is the 3rd person singular form of vzniknout.

Here it means something like:

• will arise
• will be formed
• comes into being

The subject is rozkvas, so vznikne means it will arise / be formed.

Because vzniknout is a perfective verb.

In Czech, perfective verbs use present-tense forms to express a future completed event. So:

• vznikne literally looks like a present form
• but its meaning is future: will be formed / will arise

This is very common in Czech.

Compare the idea:

• vzniká = is arising / arises repeatedly or in progress
• vznikne = will arise / will come into being once

The subject is rozkvas.

Why?

• rozkvas is in the nominative case
• z kvásku is a prepositional phrase meaning from the starter
• vznikne agrees with rozkvas: the ferment/build will be formed

So the structure is:

• Rozkvas = subject
• vznikne = verb
• z kvásku = source/origin phrase

Yes. Czech word order is fairly flexible.

For example, you could also say:

• Z kvásku vznikne rozkvas.

That would still mean the same thing, but the emphasis changes a little.

• Rozkvas vznikne z kvásku. → neutral, starting with what is being formed
• Z kvásku vznikne rozkvas. → puts more focus on from the starter

So the original sentence is natural, but not the only possible order.

Because Czech does not have articles like English a/an/the.

So:

• rozkvas can mean a ferment/build or the ferment/build
• kvásek can mean a starter or the starter

The exact meaning depends on context.

No. In this sentence, se would be wrong.

The verb is simply:

• vzniknout = to arise / come into being / be formed

So:

• Rozkvas vznikne z kvásku. = correct

English speakers sometimes expect an extra word because English often uses passive structures, but Czech does not need se here.

Even though it is written z, before the voiceless k it is usually pronounced like s.

So z kvásku sounds approximately like:

• s kvásku

This is due to normal Czech sound assimilation in pronunciation. The spelling stays z, but the sound changes.