Om to uger er der premiere, og scenen bliver sikkert fyldt med blomster bagefter.

In Danish, om + time expression is commonly used for something that will happen after that amount of time.

• om to uger = in two weeks
• om en time = in an hour
• om tre dage = in three days

So om here does not mean about. It means something like after / in when talking about future time.

Because uger is the plural of uge.

• en uge = a week
• to uger = two weeks

After numbers greater than one, Danish uses the plural noun:

• tre dage = three days
• fire måneder = four months

So to uger is just the normal plural form.

This is because Danish is a V2 language. That means the finite verb normally comes in the second position in main clauses.

Here, the sentence starts with Om to uger. Since that element is placed first, the verb must come next:

• Om to uger er der premiere.

If you start with der, then you get:

• Der er premiere om to uger.

Both are correct, but the word order changes because of the V2 rule.

A useful pattern:

• I morgen kommer han.
• Han kommer i morgen.

The verb stays in second position in both.

Here der is an expletive or dummy subject, like there in English there is / there are.

So:

• der er premiere = there is a premiere
• der kommer mange mennesker = there are many people coming / many people will come

This der does not refer to a place. It is not the same as there meaning over there.

In Danish, der er premiere is a very common fixed expression meaning there is a premiere / it’s premiere night / it opens.

So even though premiere is normally a countable noun:

• en premiere = a premiere
• premieren = the premiere

in this kind of expression, Danish often leaves out the article.

You can think of der er premiere as a set phrase. Similar event expressions are often article-less in Danish.

Scenen is the definite form of scene.

• en scene = a stage / a scene
• scenen = the stage

Danish often adds the definite article to the end of the noun instead of using a separate word like English the.

So:

• en bog = a book
• bogen = the book

Here it is scenen because the sentence refers to the stage, a specific one.

This is a passive-style construction.

• bliver = becomes / gets / is
• fyldt = filled
• med blomster = with flowers

So scenen bliver fyldt med blomster means:

• the stage gets filled with flowers
• or more naturally in English, the stage will be filled with flowers

This is built with blive + past participle, which is a common way to make the passive in Danish.

Compare:

• De fylder scenen med blomster. = They fill the stage with flowers.
• Scenen bliver fyldt med blomster. = The stage is/gets filled with flowers.

This is an important distinction.

• bliver fyldt focuses on the process or event: the stage gets filled
• er fyldt describes the resulting state: the stage is full / is filled

So:

• Scenen bliver fyldt med blomster. = the filling happens
• Scenen er fyldt med blomster. = the stage is already in that filled state

In your sentence, bliver fyldt fits because it is describing what will happen after the premiere.

Here sikkert means probably or most likely.

So:

• bliver sikkert fyldt = will probably be filled

Even though sikker often relates to sure / certain / safe, the adverb sikkert is very often used to mean probably in everyday Danish.

Examples:

• Han kommer sikkert senere. = He’ll probably come later.
• Det er sikkert rigtigt. = That is probably true.

So do not always translate sikkert as certainly. In many contexts, probably is better.

In Danish, sentence adverbs like sikkert often come in the middle of the verbal phrase.

Here the structure is:

• scenen = subject
• bliver = finite verb
• sikkert = adverb
• fyldt med blomster = rest of predicate

So:

• Scenen bliver sikkert fyldt med blomster.

This placement is very natural in Danish. Compare:

• Han kommer sikkert i morgen.
• Det bliver sikkert godt.

Bagefter means afterwards / afterward / later on.

In this sentence, it tells you when the stage will probably be filled with flowers: after the premiere.

• bagefter = afterwards
• lige bagefter = right afterwards

It often comes near the end of the clause:

• Vi spiser bagefter. = We’ll eat afterwards.
• Han kom bagefter. = He came afterwards.

So med blomster bagefter is natural Danish word order.

Because fylde noget med noget means to fill something with something.

So:

• fylde scenen med blomster = fill the stage with flowers
• glasset er fyldt med vand = the glass is filled with water

The preposition med is the normal one used with fylde in this meaning.

Yes. Danish often allows different word orders depending on what you want to emphasize, as long as the V2 rule is respected.

For example:

• Der er premiere om to uger, og scenen bliver sikkert fyldt med blomster bagefter.

This means the same as the original sentence, but the original version puts stronger focus on Om to uger.

So the original sentence is especially natural if the timing is the important new information.