Po obědě dám omáčku do mrazáku, aby byla zítra ještě dobrá.

Because the preposition po requires the locative case here.

• oběd = lunch
• po obědě = after lunch

This is a very common Czech pattern:

• po práci = after work
• po škole = after school
• po večeři = after dinner

So po obědě is not a special fixed phrase; it is just po + locative.

Here dám means I will put / I will give, so it refers to the future.

That is because dát is a perfective verb. In Czech, perfective verbs use present-tense forms to express a future completed action.

• dám = I will put / I will give
• not I am putting

In this sentence, dám omáčku do mrazáku means I’ll put the sauce into the freezer.

If you wanted an ongoing present meaning, Czech would normally use an imperfective verb such as dávám in a suitable context.

Czech often leaves out subject pronouns when they are obvious from the verb form.

• dám already means I will put
• the ending tells you the subject is I

So:

• Po obědě dám omáčku do mrazáku = After lunch, I’ll put the sauce in the freezer
• Já dám omáčku do mrazáku is possible, but já is usually added only for emphasis, contrast, or clarity

English needs I much more often than Czech does.

Because omáčku is the accusative singular of omáčka.

Here, the sauce is the direct object of dám:

• omáčka = sauce
• dám omáčku = I’ll put the sauce

This is a very common pattern in Czech: the thing directly affected by the verb goes into the accusative.

For feminine nouns ending in -a, the accusative singular is often -u:

• polévka → polévku
• káva → kávu
• omáčka → omáčku

Because do mrazáku expresses movement into the freezer, while v mrazáku expresses location inside the freezer.

• dát něco do mrazáku = to put something into the freezer
• něco je v mrazáku = something is in the freezer

So the contrast is:

• Dám omáčku do mrazáku. = I’ll put the sauce into the freezer.
• Omáčka je v mrazáku. = The sauce is in the freezer.

This is a very important Czech distinction:

• do
  • genitive = motion into
• v
  • locative = being inside

Because the preposition do takes the genitive case.

The dictionary form is:

• mrazák = freezer

After do, it becomes:

• do mrazáku = into the freezer

So this is just case grammar:

• do domu = into the house
• do obchodu = into the shop
• do mrazáku = into the freezer

Aby introduces a purpose clause. Here it means something like:

• so that
• in order that

So:

• Po obědě dám omáčku do mrazáku, aby byla zítra ještě dobrá.
• After lunch I’ll put the sauce in the freezer so that it will still be good tomorrow.

This is one of the most common uses of aby in Czech.

This is one of the most confusing things for learners: after aby, Czech uses a form that looks like the past tense, but here it is not actually past tense.

• aby byla = so that it would be / so that it will be

In purpose clauses, aby is followed by this l-participle form:

• byl for masculine
• byla for feminine
• bylo for neuter
• byli/byly for plural

Here the subject is omáčka, which is feminine singular, so we get:

• aby byla

So although byla can mean was in other contexts, here after aby it means be in the sense of purpose/result:

• so that it will be / would be

Because dobrá agrees with omáčka, which is the subject of byla in the subordinate clause.

In the clause:

• aby byla zítra ještě dobrá

the implied subject is omáčka. Since omáčka is:

• feminine
• singular

the adjective must also be:

• feminine
• singular

So:

• dobrý = basic dictionary form
• dobrá = feminine singular

This is the same kind of agreement as in:

• Polévka je dobrá. = The soup is good.
• Omáčka byla dobrá. = The sauce was good.

Here ještě means still.

So:

• zítra ještě dobrá = still good tomorrow

In other contexts, ještě can also mean yet, more, or another, but here still is the natural meaning.

Examples:

• Ještě spí. = He’s still sleeping.
• Ještě nevím. = I still don’t know.
• ještě dobrá = still good

Yes, Czech word order is flexible, and zítra can often move.

The original version:

• aby byla zítra ještě dobrá

sounds natural and neutral.

You could also hear:

• aby byla ještě zítra dobrá
• aby zítra byla ještě dobrá

These versions are all understandable, but the placement changes the emphasis a little.

Very roughly:

• zítra ještě dobrá focuses naturally on tomorrow
• ještě zítra dobrá emphasizes still tomorrow
• zítra byla ještě dobrá may sound more marked or context-dependent

Czech word order is often about information focus, not just grammar.

Because aby byla zítra ještě dobrá is a subordinate clause.

Czech normally separates subordinate clauses with a comma:

• Po obědě dám omáčku do mrazáku, aby byla zítra ještě dobrá.

This is similar to English punctuation in sentences like:

• I’ll freeze the sauce so that it will still be good tomorrow.

In Czech, the comma before words like aby, že, když, protože is very common.

Not with the same meaning.

• aby = so that / in order that → purpose
• protože = because → reason

This sentence is about the speaker’s goal:

• I’ll put the sauce in the freezer so that it will still be good tomorrow.

If you used protože, the meaning would shift toward explanation rather than purpose, and it would not fit the original idea as well.

So aby is the correct choice here because the freezer action is done for a purpose.

Mrazák means freezer.

A fridge is:

• lednice

So:

• do mrazáku = into the freezer
• do lednice = into the fridge

That matters here, because freezing something is a stronger way of keeping it good until tomorrow than just putting it in the fridge.

It can suggest several related ideas depending on context:

• still good
• still okay
• still fine to eat
• not spoiled yet

In a sentence about food, dobrá often does not only mean delicious. It can also mean the food is still in good condition.

So here the likely meaning is:

• the sauce will still be okay / still good to eat tomorrow

Yes, po obědu is also used and is very common in modern Czech.

You may encounter both:

• po obědě
• po obědu

Both mean after lunch.

For learners, the important thing is that po requires a non-nominative case here, and both of these forms are accepted in real usage. If your course or textbook prefers one, follow that model, but don’t be surprised to hear the other one.