Kvásek musí vykynout.

Kvásek is in the nominative singular, because it is the subject of the sentence — the thing that must do the action.

• kvásek = the starter / leaven
• musí = must / has to
• vykynout = to rise fully

So grammatically, kvásek is simply the noun acting as the subject.

Czech normally does not use articles like English the and a/an.

So Kvásek musí vykynout can mean:

• The starter must rise
• A starter must rise
• The sourdough starter has to rise

Which one sounds best depends on the context. Czech leaves that information unstated unless something else makes it clear.

Musí is the 3rd person singular present tense form of the verb muset (to have to, must).

Because kvásek is singular, the verb is singular too:

• já musím = I must
• ty musíš = you must
• on / ona / ono musí = he / she / it must
• kvásek musí = the starter must

So musí agrees with the singular subject kvásek.

After modal verbs like muset (must / have to), Czech usually uses the infinitive of the main verb.

So the pattern is:

• muset + infinitive

Examples:

• Musím jít. = I must go.
• Musí čekat. = He/She/It must wait.
• Kvásek musí vykynout. = The starter must rise.

This is similar to English must + base verb.

This is mainly a question of aspect and nuance.

• kynout = imperfective: to be rising, to rise as a process
• vykynout = perfective: to rise fully, to finish rising

In this sentence, vykynout suggests that the starter needs to reach the result — not just be in the middle of rising, but complete the rising.

So:

• musí kynout = it must be rising / it needs to rise as a process
• musí vykynout = it must rise properly / it must finish rising

Yes, you can, but the meaning shifts slightly.

• Kvásek musí kynout.
  This focuses more on the process of rising.

• Kvásek musí vykynout.
  This focuses more on the completed result: it has to rise enough / fully.

In baking instructions, the perfective vykynout is very natural when the speaker means that the starter must be allowed to rise before the next step.

In many Czech verbs, a prefix changes the meaning and often also the aspect.

Here, vy- helps create a perfective verb:

• kynout = to rise
• vykynout = to rise fully, to finish rising

A useful rough idea is that vy- often suggests something like out, up to completion, or to a finished state, although the exact meaning depends on the verb.

So in vykynout, the important point is the sense of completion.

Kvásek is a masculine inanimate noun.

You can tell it is masculine from its form and from how it behaves with adjectives and past-tense verbs.

For example:

• ten kvásek = this/that starter
• dobrý kvásek = a good starter

Because it is inanimate, it follows masculine inanimate patterns, not masculine animate ones.

A simple pronunciation guide is:

KVAA-sek MOO-zhee VIH-kih-nout

More carefully:

• Kvásek ≈ KVAA-sek
• musí ≈ MOO-zhee
• vykynout ≈ VIH-kih-noht or VIH-kih-nout

A few helpful points:

• Czech stress is usually on the first syllable of each word:
  • KVÁ-sek
  • MUsí
  • VY-kynout
• á and í are long vowels.
• ou is a diphthong, somewhat like the o
  • u glide in English.

The most neutral word order here is:

Kvásek musí vykynout.

Czech word order is fairly flexible, but different orders can change the emphasis.

For example:

• Kvásek musí vykynout.
  Neutral: the starter must rise.

• Musí kvásek vykynout?
  This can be a question: Does the starter have to rise?

• Vykynout musí kvásek.
  This sounds marked and puts strong emphasis on kvásek or on the action.

So yes, other word orders are possible, but the original sentence is the most natural neutral version.

Grammatically, musí is present tense, but the whole sentence often refers to a required future result.

So Kvásek musí vykynout means something like:

• The starter must rise
• The starter has to rise
• The starter needs to rise before the next step

This is very common in Czech: a present-tense modal plus an infinitive can describe a present obligation about something that will happen next.