Ballonerne skulle gerne blive hængende hele aftenen, selv hvis vinden bliver stærkere.

Because ballonerne is the definite plural form, meaning the balloons.

The forms are:

• en ballon = a balloon
• ballonen = the balloon
• balloner = balloons
• ballonerne = the balloons

Unlike English, Danish usually marks definiteness by adding an ending to the noun instead of using a separate word like the.

Here skulle gerne means something like:

• should hopefully
• ought to
• are expected to

So Ballonerne skulle gerne blive hængende... means the speaker expects or hopes that the balloons will stay up.

This is a very common Danish way to sound a bit softer and less absolute. It is not really giving an order to the balloons. It is more like a practical expectation.

Not really in the usual English sense.

Even though skulle is historically the past form of skal, in modern Danish it is often used modally, not as a simple past tense. In this sentence it does not mean that the action happened in the past. It expresses expectation, intention, or probability.

So you should not read it as the balloons were supposed to in a strictly past-time way. Here it is more like the balloons should, hopefully, stay up.

On its own, gerne often means gladly or with pleasure, but with modal verbs it often has a softer meaning related to preference, hope, or expectation.

In skulle gerne, gerne helps make the statement less blunt and more like:

• should ideally
• should hopefully
• would preferably

So it adds a tone of hopeful expectation rather than willingness.

Because after a modal verb like skulle, Danish uses the bare infinitive.

So:

• skulle blive = should stay / should become
• not skulle at blive

This is similar to English:

• should stay
• not should to stay

Common Danish modal verbs that work this way include:

• kunne
• ville
• måtte
• skulle
• burde

Blive hængende means stay hanging or more naturally here stay up.

Literally:

• blive = become / remain
• hængende = hanging

Together, they form an idiomatic expression meaning that something continues in a hanging position.

So in context:

• Ballonerne skulle gerne blive hængende hele aftenen
  = The balloons should hopefully stay up all evening

This pattern is common in Danish:

• blive stående = remain standing
• blive siddende = remain seated
• blive liggende = remain lying

Hele aftenen means the whole evening or all evening.

In Danish, when you talk about the entirety of a specific time period, you normally use hele plus the definite form:

• hele dagen = all day / the whole day
• hele natten = all night
• hele ugen = all week
• hele aftenen = all evening

So hele aften would not be the normal form here.

Selv hvis means even if.

It introduces a hypothetical condition that would not change the result in the main clause.

So:

• selv hvis vinden bliver stærkere
  = even if the wind gets stronger

This tells you that the balloons are expected to stay up regardless of that possible change.

They are similar, but not the same.

• selv hvis = even if
  This is conditional and hypothetical.
• selvom = although / even though
  This usually refers to something treated as a fact.

In this sentence, selv hvis is right because the wind becoming stronger is presented as a possibility, not as an established fact.

Compare:

• Selv hvis vinden bliver stærkere, bliver ballonerne hængende.
  = Even if the wind gets stronger, the balloons will stay up.

• Selvom vinden bliver stærkere, bliver ballonerne hængende.
  = Even though the wind is getting stronger, the balloons will stay up.

Vinden is the definite singular form, meaning the wind.

The forms are:

• en vind = a wind
• vinden = the wind

In this sentence, the speaker is referring to the wind in the situation they are talking about, so the definite form sounds natural.

Because bliver stærkere means gets stronger or becomes stronger, which focuses on a change.

• bliver stærkere = becomes stronger
• er stærkere = is stronger

Here the idea is that the wind may increase in strength during the evening, so bliver stærkere is the natural choice.

Also, stærkere is the comparative form of stærk:

• stærk = strong
• stærkere = stronger
• stærkest = strongest

The main clause is:

• Ballonerne skulle gerne blive hængende hele aftenen

The subordinate clause is:

• selv hvis vinden bliver stærkere

In the main clause, Danish follows the usual verb-second pattern:

• Ballonerne = subject
• skulle = finite verb
• gerne blive hængende hele aftenen = rest of the clause

In the subordinate clause, the word order is more straightforward:

• vinden = subject
• bliver = verb
• stærkere = complement

If you move the subordinate clause to the front, the main clause still follows Danish verb-second order:

• Selv hvis vinden bliver stærkere, skulle ballonerne gerne blive hængende hele aftenen.

Notice that skulle comes before ballonerne in that version. That inversion is very typical in Danish main clauses.

It is acceptable and very common to write a comma there because selv hvis vinden bliver stærkere is a subordinate clause.

So:

• Ballonerne skulle gerne blive hængende hele aftenen, selv hvis vinden bliver stærkere.

is perfectly normal.

In Danish comma usage, you may also see variation depending on whether someone uses start comma conventions or not, but the sentence as written looks completely natural.