Marináda je vedle grilovací omáčky.

Because vedle usually requires the genitive case in Czech.

• Dictionary form: omáčka = sauce
• After vedle = vedle omáčky = next to the sauce

So omáčky is the genitive singular form of omáčka.

For feminine nouns like omáčka, the ending often changes like this:

• nominative: omáčka
• genitive: omáčky

That is why the sentence says vedle grilovací omáčky, not vedle grilovací omáčka.

Vedle means next to / beside.

In this meaning, it is used with the genitive:

• vedle domu = next to the house
• vedle auta = next to the car
• vedle grilovací omáčky = next to the barbecue sauce

So when you see vedle, a good question to ask yourself is: What form does the following noun need in the genitive?

Because grilovací describes what kind of sauce it is: barbecue / grilling sauce.

• omáčka = sauce
• grilovací omáčka = barbecue sauce / grilling sauce

Since omáčka is in the genitive singular after vedle, the adjective must match it:

• nominative: grilovací omáčka
• genitive: grilovací omáčky

In Czech, adjectives agree with the noun in gender, number, and case.

It is an adjective.

It comes from grilovat = to grill, and grilovací means something like:

• grilling
• for grilling
• sometimes naturally translated as barbecue

Examples:

• grilovací omáčka = barbecue sauce
• grilovací koření = grilling spice / barbecue seasoning
• grilovací sezóna = grilling season

So in your sentence, grilovací simply modifies omáčky.

Je is the 3rd person singular form of být = to be.

• Marináda je vedle grilovací omáčky.
• The marinade is next to the barbecue sauce.

Czech often uses být in sentences that describe location:

• Kniha je na stole. = The book is on the table.
• Auto je před domem. = The car is in front of the house.

So je here means is.

Normally, in standard Czech, no.

You usually need je here because this is a normal present-tense sentence with to be:

• Marináda je vedle grilovací omáčky.

Leaving out je would sound incomplete or unnatural in standard Czech.

Czech sometimes drops forms of to be in very informal speech or in special styles, but for learners, you should keep je here.

Because Marináda is the subject of the sentence, so it is in the nominative case.

The sentence answers the question:

• Co je vedle grilovací omáčky?
• Marináda.

So:

• marináda = nominative singular
• omáčky = genitive singular after vedle

This is a very common Czech pattern: subject in nominative, noun after a preposition in the case required by that preposition.

Yes. Czech word order is fairly flexible.

The neutral version is:

• Marináda je vedle grilovací omáčky.

But you could also say:

• Vedle grilovací omáčky je marináda.

Both mean essentially the same thing. The difference is mostly about focus or emphasis:

• Marináda je vedle grilovací omáčky.
  Focuses more naturally on marináda as the topic.
• Vedle grilovací omáčky je marináda.
  Starts with the location, so it can sound like Next to the barbecue sauce, there is a marinade.

For learners, the original sentence is a perfectly good basic word order.

A rough pronunciation guide for an English speaker is:

• Marináda ≈ mah-rih-NAA-dah
• je ≈ yeh
• vedle ≈ VED-leh
• grilovací ≈ grih-loh-VAA-tsee
• omáčky ≈ oh-MAACH-kee

A few useful pronunciation notes:

• j in Czech sounds like English y in yes
• č sounds like ch in chocolate
• á is a long a
• í is a long ee

So omáčky is not pronounced like English -ky in sky; it is more like -kee.

They are related food words, but not the same thing.

• marináda = marinade
  A liquid or mixture used to soak food before cooking
• omáčka = sauce
  Something served with food or added as a topping/condiment

So in the sentence:

• Marináda = the marinade
• grilovací omáčka = the barbecue sauce

In real life, some products might blur the distinction, but grammatically and basically, they are different words.

It is a very natural and common way, but not the only possible wording.

Common options include:

• grilovací omáčka = grilling/barbecue sauce
• barbecue omáčka or BBQ omáčka = also possible, especially on packaging or in informal usage

For a learner, grilovací omáčka is a good standard Czech expression.

Because some Czech adjectives have endings that look the same in more than one case.

Here you have:

• nominative feminine singular: grilovací omáčka
• genitive feminine singular: grilovací omáčky

The visible change is mainly on the noun omáčka → omáčky, while grilovací stays the same in form.

That is normal. Not every adjective shows a big visible change in every case.

No. It often means physical next to / beside, but it can also be used more abstractly.

Examples:

• Dům je vedle školy. = The house is next to the school.
• Vedle práce studuje. = Besides work, he/she studies.
• Vedle češtiny se učí i polsky. = Besides Czech, he/she is also learning Polish.

In your sentence, though, the meaning is clearly physical location: the marinade is next to the barbecue sauce.