Der Reparaturversuch gelingt am Veranstaltungstag.

Reparaturversuch is a compound noun:

• Reparatur = repair
• Versuch = attempt, try

So Reparaturversuch means repair attempt.

German very often combines nouns into one word where English would use two or more words. In compounds, the last part is the main noun, so this is literally an attempt of repair.

Because Versuch is masculine, the whole compound is masculine too, which is why it takes der.

The noun Reparaturversuch is masculine, so in the nominative singular it takes der:

• der Reparaturversuch

In this sentence, der Reparaturversuch is the subject, so nominative is needed.

A useful rule for compounds: the gender of a compound noun is determined by the final element.

Since Versuch is masculine, Reparaturversuch is also masculine.

The infinitive is gelingen.
Here it is conjugated for third person singular, because the subject is singular:

• der Reparaturversuch = singular
• therefore: gelingt

So:

• der Reparaturversuch gelingt = the repair attempt succeeds

Compare:

• Der Versuch gelingt.
• Die Versuche gelingen.

Singular subject → gelingt
Plural subject → gelingen

Gelingen means something like to succeed, to turn out well, or to be successful.

But it is used differently from English to succeed. In German, the thing that succeeds is often the grammatical subject.

So in this sentence:

• Der Reparaturversuch gelingt.
• literally: The repair attempt succeeds.

Another very common pattern is:

• jemandem gelingt etwas
• something succeeds for someone
• natural English: someone manages to do something or something works out for someone

Example:

• Dem Techniker gelingt die Reparatur.
• The technician succeeds in the repair / The technician manages the repair.

So gelingen often goes with:

• the successful thing in nominative
• the person in dative, if mentioned

Because German does not have to mention the person with gelingen.

This sentence focuses on the repair attempt itself:

• Der Reparaturversuch gelingt am Veranstaltungstag.

That means the important information is that the attempt is successful, not who does it.

If you want to add a person, German usually uses the dative:

• Dem Techniker gelingt der Reparaturversuch am Veranstaltungstag.

So the original sentence is completely natural even without naming the person.

Am is a contraction of:

• an dem → am

So:

• am Veranstaltungstag = on the event day / on the day of the event

Here an is used in a time expression, and it takes the dative in this sentence:

• dem Veranstaltungstag → contracted to am Veranstaltungstag

This is very common in German:

• am Montag = on Monday
• am Wochenende = on the weekend
• am Veranstaltungstag = on the day of the event

Because it follows the preposition an in a time expression.

With an, German can use either accusative or dative depending on meaning, but here it is about when something happens, not movement toward somewhere. So German uses the dative:

• an dem Veranstaltungstag
• contracted: am Veranstaltungstag

So the case is not caused by the noun itself, but by the preposition an in this context.

Yes. German word order is flexible, but the finite verb still has to stay in second position in a normal main clause.

The original sentence is:

• Der Reparaturversuch gelingt am Veranstaltungstag.

You can also say:

• Am Veranstaltungstag gelingt der Reparaturversuch.

Both are correct. The second version emphasizes when it happens.

What you cannot do in a normal statement is put the verb later like in English:

• Der Reparaturversuch am Veranstaltungstag gelingt.
  This is possible only in special contexts, not as the normal neutral sentence.

So the main rule is: one element first, conjugated verb second.

Because they are nouns, and in German all nouns are capitalized.

So:

• der Reparaturversuch
• der Veranstaltungstag

This is one of the most visible spelling differences between German and English.

That would sound unnatural in normal German.

A better match for the original structure is:

• Der Reparaturversuch gelingt am Veranstaltungstag.

If you want to use erfolgreich, you would usually say something like:

• Der Reparaturversuch ist am Veranstaltungstag erfolgreich.

But even that is less natural in many contexts than gelingt.

So gelingen is the more idiomatic choice when you mean that an attempt or action works out successfully.

They are related, but not interchangeable in all situations.

• gelingen: focuses on the fact that something turns out well

  • Der Versuch gelingt.

• schaffen: focuses more on managing to do something

  • Er schafft die Reparatur.
  • He manages the repair / gets it done.

• erfolgreich sein: describes a person or thing as successful

  • Die Reparatur ist erfolgreich.

For this sentence, gelingen is especially natural because the subject is the repair attempt, and the sentence says that this attempt succeeds.