Dieses Buch ist langweilig, ich möchte ein anderes.

Because Buch is a neuter noun in German: das Buch.

The word dies- changes its ending to match the noun’s:

• dieser for masculine
• diese for feminine
• dieses for neuter

Here, Buch is neuter and is the subject of the clause, so German uses dieses Buch.

It is in the nominative case because it is the subject of ist langweilig.

You can think of it as:

• Dieses Buch = the thing being described
• ist langweilig = is boring

So dieses Buch is the subject, and subjects are in the nominative.

Because it comes after the verb ist and works as a predicate adjective.

In German, adjectives:

• before a noun usually take endings: ein langweiliges Buch
• after sein, werden, bleiben, etc. do not take endings: Das Buch ist langweilig

So:

• ein langweiliges Buch = a boring book
• Das Buch ist langweilig = the book is boring

Because the sentence contains two main clauses:

1. Dieses Buch ist langweilig
2. ich möchte ein anderes

German can join main clauses with a comma, especially in short, natural sentences like this.

Other possible punctuation would also work:

• Dieses Buch ist langweilig. Ich möchte ein anderes.
• Dieses Buch ist langweilig; ich möchte ein anderes.

You could also add und:

• Dieses Buch ist langweilig, und ich möchte ein anderes.

But the version without und sounds direct and natural.

möchte is softer and more polite than will.

• ich möchte = I would like
• ich will = I want

In many situations, ich will can sound stronger, more blunt, or more forceful than English I want. So when talking about preferences, German often uses möchte.

In this sentence, ich möchte ein anderes sounds natural and polite: I’d like another one.

möchte is the usual polite/desire form of mögen.

Learners often first meet mögen as to like, but möchte is very commonly used to mean would like.

Examples:

• Ich möchte Kaffee.
• Ich möchte gehen.
• Ich möchte ein anderes.

So even though it comes from mögen, you should often think of möchte as its own very common expression meaning would like.

Because it means another one or more literally another one/book.

The full version would be:

• ich möchte ein anderes Buch

But since Buch was already mentioned, German leaves it out:

• Dieses Buch ist langweilig, ich möchte ein anderes.

English does the same:

• This book is boring, I want another.
• This book is boring, I want another one.

Because it refers back to Buch, which is neuter singular, and it is the object of möchte, so it is in the accusative.

The hidden full phrase is:

• ein anderes Buch

Since Buch is neuter, the form is:

• ein anderes Buch

And when Buch is omitted, the form stays:

• ein anderes

So the -es matches a neuter singular noun.

Because here andere is being used in a pronoun-like way, meaning another one, and German writes it lowercase in this kind of use.

So:

• ich möchte ein anderes = I’d like another one

Even though the noun is omitted, German does not capitalize anderes here.

Yes, absolutely.

Both are correct:

• ich möchte ein anderes
• ich möchte ein anderes Buch

The shorter version is more natural when it is already obvious that you mean book. The longer version is more explicit and can help if there might be any confusion.

Because the missing noun is Buch, and Buch is neuter, not masculine.

Compare:

• Ich möchte ein anderes Buch.
• Ich möchte einen anderen Roman.

Why the difference?

• Buch is neuter → ein anderes
• Roman is masculine → einen anderen

So the form changes according to the gender of the noun being referred to.

Yes.

• Das Buch ist langweilig = The book is boring
• Dieses Buch ist langweilig = This book is boring

dieses points more clearly to a specific book and often feels a bit more contrastive, as if you are choosing among books or reacting to one in front of you.

So both are possible, but dieses Buch is more explicitly this book.

Because the second part is a new main clause, and main clauses in German normally follow the verb-second rule.

In ich möchte ein anderes:

• ich is in the first position
• möchte is in the second position

So the structure is completely normal.

You could say möchte ich only if something else comes first for emphasis, for example:

• Ein anderes möchte ich.

That is grammatical, but much less neutral. The standard version is:

• ich möchte ein anderes