Vi planlægger en overraskelsesfest igen til sommer, hvis hun ikke opdager noget i forvejen.

Because Danish, like German, often forms compound nouns by writing them as one word.

So:

• overraskelse = surprise
• fest = party

Together:

• overraskelsesfest = surprise party

The -s- in the middle is a common linking sound in Danish compounds. You just have to learn it as part of the compound form.

This is very normal in Danish:

• fødselsdag = birthday
• sommerferie = summer holiday
• arbejdsdag = workday

English often writes similar ideas as separate words, but Danish usually prefers one compound word.

Danish very often uses the present tense for a planned or expected future action, just like English can do in sentences such as We’re leaving tomorrow.

So:

• Vi planlægger ... til sommer
  literally: We plan ... in summer natural meaning: We are planning ... for the summer / next summer

If the future is already clear from words like til sommer, Danish does not need a separate future form.

You can use vil for future in Danish, but it is not always necessary.

Til sommer means in the summer or more naturally this coming summer / next summer, depending on context.

It often refers to the summer that is coming up relative to when the speaker is talking.

Similar expressions:

• til vinter = this coming winter / in winter
• til efteråret = this coming autumn
• til foråret = this coming spring

So igen til sommer means something like:

• again this coming summer
• again next summer

In this sentence, igen means again, and it is placed where it modifies the whole idea of planning the party again.

• Vi planlægger en overraskelsesfest igen til sommer

This sounds natural in Danish. The adverb placement can shift a little depending on what you want to emphasize, but this version is straightforward and idiomatic.

Compare:

• Vi planlægger igen en overraskelsesfest til sommer
  possible, but less natural in many contexts
• Vi planlægger en overraskelsesfest til sommer igen
  can sound as if til sommer is what is being repeated

So the original word order is a natural way to say that the whole event is happening again.

Because hvis is used for a condition:

• hvis hun ikke opdager noget = if she doesn’t notice/find out anything

Use om when if means whether:

• Jeg ved ikke, om hun kommer = I don’t know if/whether she is coming

So:

• hvis = if, in a conditional sense
• om = whether / if, in an indirect question

In your sentence, it is clearly conditional, so hvis is correct.

Because this is a subordinate clause introduced by hvis.

In Danish subordinate clauses, words like ikke usually come before the finite verb.

So:

• hvis hun ikke opdager noget

Structure:

• hvis = subordinator
• hun = subject
• ikke = negation
• opdager = verb
• noget = object

This is different from a main clause, where Danish normally has verb second word order:

• Hun opdager ikke noget = main clause
• ... hvis hun ikke opdager noget = subordinate clause

That contrast is very important in Danish grammar.

It can cover several related meanings:

• discover
• notice
• find out

In this sentence, the most natural English idea is probably:

• if she doesn’t find out beforehand
• or if she doesn’t notice anything beforehand

The verb opdage often means becoming aware of something, especially something hidden or previously unknown.

So here it suggests that the surprise party will only work if she does not realize what is being planned.

Because in Danish, noget is often used in negative sentences to mean anything.

So:

• hun opdager noget = she notices something
• hun ikke opdager noget = she doesn’t notice anything

This is very similar to English:

• something in positive statements
• anything in negative statements and many questions

Ingenting means nothing, but using it here would create a different structure:

• hun opdager ingenting = she notices nothing

That is possible, but ikke ... noget is very common and natural:

• hun ikke opdager noget

I forvejen means:

• beforehand
• in advance
• ahead of time

So:

• hvis hun ikke opdager noget i forvejen = if she doesn’t find out anything beforehand

It is a very useful fixed expression in Danish.

Examples:

• Jeg vidste det i forvejen = I knew it beforehand
• Bestil i forvejen = Order in advance

Because fest is a common gender noun in Danish, so it takes en.

• en fest
• therefore en overraskelsesfest

Danish nouns are either:

• common gender → en
• neuter → et

Since the last part of the compound is fest, the whole compound keeps that gender:

• en fest
• en overraskelsesfest

This is a useful rule: in Danish compounds, the final noun determines the gender.

A learner often finds this word intimidating because it is long, but it helps to break it up:

• o-ver-ras-kel-ses-fest

A few useful points:

• Danish pronunciation is usually less fully pronounced than the spelling suggests
• some unstressed syllables become weaker
• the r sounds and vowels may be quite different from what an English speaker expects

You do not need to pronounce every written syllable very sharply. It is better to listen to native audio and imitate the rhythm.

It may help to practice it in chunks:

• en overraskelse
• en fest
• en overraskelsesfest

Yes, in many contexts you could say:

• hvis hun ikke finder noget ud af i forvejen
• or more naturally
• hvis hun ikke finder ud af det i forvejen

But the meaning shifts slightly.

• opdager noget focuses on noticing/discovering something
• finder ud af det focuses more on finding out / learning the truth

In your original sentence, opdager noget works well because the idea is that she must not become aware that something is being planned.

The sentence has:

1. a main clause
   
   • Vi planlægger en overraskelsesfest igen til sommer
2. a subordinate clause introduced by hvis
   
   • hvis hun ikke opdager noget i forvejen

So the overall pattern is:

• Main clause + conditional clause

Very literally:

• We plan a surprise-party again this-coming-summer, if she not discovers anything beforehand.

Natural English:

• We’re planning another surprise party this summer, if she doesn’t find out beforehand.

This is a very common Danish sentence pattern:

• [main clause], hvis [condition]

For example:

• Vi tager afsted, hvis vejret er godt = We’ll leave if the weather is good
• Jeg kommer, hvis jeg kan = I’ll come if I can