Die Spielanleitung nennt den Startspieler.

Because Die Spielanleitung is the subject of the sentence — the thing doing the action.

• die Spielanleitung = the game instructions / the instruction booklet
• nennt = names / states / specifies
• den Startspieler = the starting player

So grammatically, the sentence says that the instructions are the thing that name or specify the starting player.

Because Spielanleitung is a feminine noun, and here it is in the nominative case as the subject.

Its article in the singular is:

• die Spielanleitung = nominative
• der Spielanleitung = genitive or dative, depending on context

In this sentence, it is the subject, so nominative is needed: die Spielanleitung.

Because Startspieler is the direct object of the verb nennen, so it takes the accusative case.

• nominative: der Startspieler
• accusative: den Startspieler

So:

• Der Startspieler nennt ... = the starting player names ...
• ... nennt den Startspieler = ... names the starting player

The article changes from der to den because masculine nouns change in the accusative.

Here, nennt means something like:

• names
• states
• specifies
• designates

The basic verb is nennen.

In many contexts, nennen can mean to call someone or something by a name. But in this sentence, it is more like to identify or to specify who the starting player is.

So the sense is not necessarily that the instructions are giving someone a nickname. It is more that they are indicating who starts.

Because standard German main clauses usually follow the verb-second rule.

In this sentence:

• 1st position: Die Spielanleitung
• 2nd position: nennt
• then: den Startspieler

That is a normal German main clause pattern.

You could also move another element to the front, but the finite verb would still stay second, for example:

• Den Startspieler nennt die Spielanleitung.

That sounds more marked or stylistically unusual, but it is grammatically possible.

German often shows this through case, not just word order.

Here:

• die Spielanleitung = nominative subject
• den Startspieler = accusative object

That tells you:

• the instructions do the naming
• the starting player is the one being named

In English, we often rely more on position. In German, the articles are a big clue.

Yes. Spielanleitung is a compound noun:

• Spiel = game
• Anleitung = instructions / guide

Together:

• Spielanleitung = game instructions / instruction booklet / game manual

German very often combines nouns into one longer word instead of using separate words like English does.

It is also a compound noun:

• Start = start
• Spieler = player

So Startspieler means starting player or first player, depending on context.

The gender comes from the final part, Spieler, which is masculine:

• der Startspieler

If you needed a feminine form for a female starting player, you might see:

• die Startspielerin

Because it is the 3rd person singular present tense form of nennen.

Conjugation:

• ich nenne
• du nennst
• er/sie/es nennt
• wir nennen
• ihr nennt
• sie/Sie nennen

Since die Spielanleitung is grammatically she/it in German and singular, the verb form is nennt.

Yes, depending on context, that can work. But Spielanleitung usually refers more specifically to:

• the instructions
• the instruction booklet
• the game manual

If the context is a board game, a natural English translation might be:

• The instructions specify the starting player.
• The rules state who starts.

So name is possible, but in natural English specify or state may sound more idiomatic.

Yes: both nouns are singular.

• die Spielanleitung = the instruction booklet / instructions
• den Startspieler = the starting player

Even though English instructions is often plural, German often uses singular die Anleitung or die Spielanleitung for a booklet/manual.

If you wanted a plural form, it would be different:

• die Spielanleitungen = the instruction booklets

But in this sentence, singular is the natural choice.

Yes. This is a very common thing that can feel a little odd to English speakers.

German often uses a singular noun where English prefers a plural expression:

• die Anleitung = the instructions / the manual
• die Bedienungsanleitung = the operating instructions / manual
• die Spielanleitung = the game instructions / rulebook

So even though the German word is singular, the English meaning may naturally be plural.

It is grammatical and understandable, but in real game-related German, people might also say things like:

• Die Spielanleitung bestimmt den Startspieler.
• In der Spielanleitung steht, wer beginnt.
• Die Spielanleitung legt fest, wer beginnt.

Your sentence is perfectly fine, but depending on context, another verb may sound a bit more natural. Nennen is still a good verb for learning because it clearly shows the structure:

• subject in nominative
• object in accusative