Der Spieleabend beginnt mit einem Kartenspiel.

Because Abend is a masculine noun in German: der Abend.
The word Spieleabend is a compound noun made from Spiele + Abend, and in German the last part of a compound determines the gender. Since Abend is masculine, the whole word is masculine too:

• der Abend
• der Spieleabend

Spieleabend is a compound noun. It combines:

• Spiele = games
• Abend = evening

So Spieleabend means something like game night or games evening.

A very important German rule is that compound nouns are written as one word, not as separate words. So German uses:

• der Spieleabend

not:

• der Spiele Abend

The -e comes from Spiele, the plural of Spiel. In compounds, German sometimes uses a plural form or a linking form before the second noun.

So:

• das Spiel = the game
• die Spiele = the games
• der Spieleabend = game night / evening of games

A learner does not always need to predict this perfectly at first; many compounds simply have to be learned as vocabulary. But here, Spieleabend is the normal form.

In a normal German main clause, the finite verb usually comes in position 2.

Here the sentence is:

• Der Spieleabend = subject
• beginnt = finite verb
• mit einem Kartenspiel = prepositional phrase

So the structure is:

• Subject + Verb + Rest

This is one of the most important basic German word order patterns.

Because beginnt is the conjugated form of the verb beginnen for er/sie/es or for a singular noun like der Spieleabend.

The verb is conjugated to match the subject:

• ich beginne
• du beginnst
• er/sie/es beginnt
• wir beginnen
• ihr beginnt
• sie/Sie beginnen

Since der Spieleabend is singular, German uses beginnt.

The preposition mit always takes the dative case in German.

The noun Kartenspiel is neuter:

• das Kartenspiel

The indefinite article for a neuter noun in the dative is einem. So:

• nominative: ein Kartenspiel
• dative: mit einem Kartenspiel

That is why the sentence says mit einem Kartenspiel.

You learn it with its article:

• das Kartenspiel

Also, Spiel is neuter:

• das Spiel

Since Kartenspiel is a compound ending in Spiel, the whole compound is also neuter. Again, the last element of the compound determines the gender.

So:

• das Spiel
• das Kartenspiel

Here mit means with.

So mit einem Kartenspiel means with a card game. In this sentence, it tells you how the game night begins: it begins with a card game.

It is not saying possession; it is describing what the event starts off with.

In German, all nouns are capitalized, not just proper names.

So in this sentence:

• Spieleabend is capitalized because it is a noun.
• Kartenspiel is capitalized because it is a noun.

This is a standard rule in German spelling and is very helpful when reading.

Yes. That is a very natural alternative.

• Der Spieleabend beginnt mit einem Kartenspiel.
• Der Spieleabend fängt mit einem Kartenspiel an.

Both mean basically the same thing: The game night begins with a card game.

A small difference in tone:

• beginnen can sound a little more neutral or formal.
• anfangen / fängt ... an is very common in everyday speech.

Usually das Kartenspiel means a card game. In some contexts, it can also mean a deck/set of playing cards, but in this sentence the meaning is clearly a card game, because it says the evening begins with one.

If German wanted to make deck of cards clearer, it might use something like:

• ein Kartenspiel Karten is not correct
• more naturally: ein Kartendeck or simply Spielkarten

So here, learners should understand Kartenspiel as a card game.

Mostly yes, but not perfectly naturally.

Word by word:

• Der = the
• Spieleabend = game night
• beginnt = begins
• mit = with
• einem Kartenspiel = a card game

So a close translation is:

• The game night begins with a card game.

That is understandable English, but depending on context, a more natural English version might be:

• Game night starts with a card game.
• The evening starts with a card game.

German and English often match closely here, but good translation still depends on natural phrasing.