Kukuřici nemusíte marinovat; stačí, když ji opečete.

Because kukuřici is in the accusative singular, not the basic dictionary form.

• kukuřice = nominative singular
• kukuřici = accusative singular

It is accusative here because it is the direct object of marinovat: you marinate the corn.

So:

• Kukuřice je sladká. = The corn is sweet.
• Marinuji kukuřici. = I am marinating the corn.

Nemusíte comes from muset, which means must / have to.

With negation, nemusíte means you do not have to or you need not. It expresses lack of necessity, not a ban.

So this sentence means:

• it is not necessary to marinate the corn

It does not mean:

• you must not marinate it

That distinction is important:

• nemusíte = you don’t have to
• nesmíte = you must not / you are not allowed to

The ending -te can refer to:

• you plural = more than one person
• formal/polite you singular = one person, politely

So nemusíte can mean either:

• you all don’t have to
• you don’t have to when speaking politely to one person

The same is true of opečete later in the sentence.

If you were speaking informally to one person, you would say:

• nemusíš
• opečeš

Because after the modal verb muset, Czech normally uses an infinitive.

So the structure is:

• nemusíte + infinitive

Here:

• nemusíte marinovat = you do not have to marinate

This is very similar to English have to + verb.

Other examples:

• Musíte čekat. = You have to wait.
• Nemusíte platit. = You don’t have to pay.

Stačí means is enough / suffices.

In this kind of sentence, Czech often uses stačí impersonally, with no clearly stated subject, much like English it’s enough.

So:

• Stačí, když ji opečete. = It is enough if you roast it.

The verb is 3rd person singular because that is the normal form in this impersonal-style construction.

A very common Czech pattern is:

• Stačí, když... = It’s enough if...
• Stačí udělat X. = It’s enough to do X.

Normally když often means when, but in this pattern after stačí, it is very often best understood as if or when you just... in English.

So:

• Stačí, když ji opečete.

is naturally translated as something like:

• It’s enough if you roast it
• You only need to roast it
• Just roast it

This is a very common Czech structure. It does not necessarily sound strongly temporal here.

Ji is the accusative singular feminine pronoun meaning her / it.

Here it refers back to kukuřici, which is a feminine noun. So instead of repeating kukuřici, Czech uses the pronoun:

• ji = it

That gives:

• když ji opečete = when/if you roast it

Without ji, the object would be missing.

Opečete comes from the perfective verb opéct. Perfective verbs focus on a completed result.

That makes sense here, because the idea is:

• you do not need to marinate it
• it is enough to roast it and get that done

By contrast:

• opékat / opékáte is imperfective and focuses more on the process or repeated action
• opéct / opečete focuses on the single completed action

Also, in Czech, the present tense of a perfective verb often refers to the future:

• opečete = you will roast it / you roast it

So here když ji opečete means roughly if/when you roast it.

Yes. This is a nice example of Czech aspect.

• marinovat is imperfective
• opéct is perfective

Why this combination?

• With nemusíte marinovat, the sentence is talking about the activity in general: marinating is not necessary.
• With ji opečete, the sentence points to a single completed action that will be enough.

So the aspect choice matches the meaning:

• marinating as a general optional process
• roasting as the one sufficient completed step

The word order is flexible.

Kukuřici nemusíte marinovat puts kukuřici at the front as the topic, something like:

• As for the corn, you don’t have to marinate it

Other possible word orders include:

• Nemusíte kukuřici marinovat.
• Nemusíte marinovat kukuřici.

These are all understandable, but they may differ slightly in emphasis.

The same idea applies to the second half. Czech could also say:

• Stačí ji opéct.

That version is shorter and very natural too.
Stačí, když ji opečete sounds a bit more conversational and personal.

The semicolon separates two closely related parts:

• Kukuřici nemusíte marinovat
• stačí, když ji opečete

It creates a stronger pause than a comma, but it still shows that the two clauses belong together closely in meaning.

So it is basically a stylistic punctuation choice:
first clause = what is not necessary,
second clause = what is enough instead.

A full stop could also work, but the semicolon keeps the connection tighter.