Learn Hardcore German: Der Ausdruck in diesem Buch ist schwierig. - The expression in this book is difficult.

Ausdruck is masculine, so its definite article in the nominative case is der.
Die is for feminine singular nouns (and all plurals).
Das is for neuter singular nouns.

Here Der Ausdruck is the subject of the sentence, so it must be in the nominative case:

Nominative (subject): der Ausdruck ist schwierig – the expression is difficult
Accusative (object): Ich verstehe den Ausdruck nicht – I don’t understand the expression
Dative (indirect object): Mit dem Ausdruck habe ich Probleme – I have problems with the expression

What exactly does Ausdruck mean here? Is it like “expression”, “phrase”, or “style”?

Ausdruck is a flexible word and its meaning depends on context. It can mean:

Facial expression / emotional expression
- Sein Ausdruck war traurig. – His expression was sad.
Linguistic expression, phrasing, choice of words – this is the meaning in your sentence
- Der Ausdruck in diesem Buch ist schwierig.
  → The phrasing / way of expressing things in this book is difficult.
Printed copy (in some contexts)
- Ich brauche einen Ausdruck der Datei. – I need a printout of the file.

In your sentence, it refers to the language style / formulations / word choice used in the book, not to a single phrase but to how the book is written overall.

Why is it in diesem Buch and not in dieses Buch?

This is about case and how the preposition in works.

The phrase in diesem Buch is describing a location (where the difficult expression/phrasing is), so in takes the dative case here.

Buch is neuter: das Buch
Dative singular neuter article (or demonstrative) is dem / diesem

So you get:

in diesem Buch = in this book (dative, location)

In dieses Buch would be accusative, which is used for movement into something:

Ich schreibe in dieses Buch. – I (am) write(ing) into this book.

Compare:

Dative (where?): Der Ausdruck in diesem Buch ist schwierig. – The phrasing in this book is difficult.
Accusative (where to?): Ich schreibe die Vokabeln in dieses Buch. – I write the vocabulary into this book.

What case is diesem Buch, and how is dies- declined in other cases?

Diesem Buch is dative singular neuter.

dies- is a demonstrative (this / that) and declines like the definite article.
Buch is neuter, so:

Singular of dies-:

Case	Masculine	Neuter	Feminine	English
Nom.	dieser	dieses	diese	this
Acc.	diesen	dieses	diese	this
Dat.	diesem	diesem	dieser	to/at this
Gen.	dieses	dieses	dieser	of this

So in your sentence:

in diesem Buch
- in → dative (because of location)
- diesem → dative singular neuter form of dies-
- Buch → neuter noun

Putting it together: in diesem Buch = in this book.

Why is in diesem Buch in the middle of the sentence? Can I move it?

Yes, you can move in diesem Buch. In German, many elements can be moved without changing the basic meaning; you mainly change the emphasis.

Your original sentence:

Der Ausdruck in diesem Buch ist schwierig.
→ neutral emphasis; we’re talking about the expression in this specific book.

You could also say:

In diesem Buch ist der Ausdruck schwierig.
→ stronger emphasis on this book (in this book, the expression is difficult – maybe in others it isn’t).

What you cannot change is the rule that the finite verb (ist) must be in second position in a main clause:

✅ Der Ausdruck | ist | in diesem Buch schwierig.
✅ In diesem Buch | ist | der Ausdruck schwierig.
❌ Der Ausdruck in diesem Buch schwierig ist. (verb not in second position)

So yes, you can move in diesem Buch, but the verb stays second.

Why does schwierig come at the end, and why doesn’t it have an ending like schwierige?

Two points here: position and adjective ending.

Position
In German main clauses, the verb is in second position, and often the rest of the information comes after it. Predicative adjectives (adjectives used with sein, werden, bleiben) normally go later in the sentence:
- Der Ausdruck ist schwierig.
- Die Aufgabe war einfach.
If there are other elements like in diesem Buch, they usually come before the adjective:
- Der Ausdruck in diesem Buch ist schwierig.
No adjective ending here
Schwierig is used predicatively (after ist) rather than directly before a noun:
- Predicative: Der Ausdruck ist schwierig. → no ending
- Attributive (before a noun): der schwierige Ausdruck → needs an ending

Adjectives only take endings when they stand before a noun:

Der schwierige Ausdruck (adjective before Ausdruck)
Der Ausdruck ist schwierig (adjective after ist, no ending)

What’s the difference between schwierig and schwer here? Which sounds more natural?

In this context, both schwierig and schwer can mean “difficult”, but there are some tendencies:

schwierig
- Slightly more formal or neutral for intellectual / abstract difficulty.
- Often used with tasks, problems, texts, situations.
- Der Ausdruck in diesem Buch ist schwierig. sounds very natural.
schwer
- Can mean heavy (physical weight) or difficult.
- In the sense of “difficult”, it’s common in spoken language and can feel a bit more colloquial, depending on context.
- Der Ausdruck in diesem Buch ist schwer. is also correct and understandable.

In written, slightly formal language about a book’s language, schwierig is usually the first choice.

Can I also say Die Ausdrücke in diesem Buch sind schwierig? What’s the difference?

Yes, that’s correct, but it changes the nuance:

Der Ausdruck in diesem Buch ist schwierig.
- Focuses on the general style / phrasing overall.
- Suggests the way the book is written as a whole is difficult.
Die Ausdrücke in diesem Buch sind schwierig.
- Focuses more on individual expressions / specific phrases in the book.
- Implies there are many particular expressions that are difficult.

Grammar changes:

Der Ausdruck … ist → singular subject + singular verb
Die Ausdrücke … sind → plural subject + plural verb

Is Ausdruck here singular or some kind of “mass” meaning like “expression in general”?

Formally, Ausdruck here is singular, but conceptually it’s often understood in a collective or abstract way:

Der Ausdruck in diesem Buch = the language / phrasing / style used in this book

It doesn’t mean just one single expression; it refers to the overall way things are expressed.

If you want to clearly talk about multiple separate expressions, you use the plural:

Die Ausdrücke in diesem Buch sind schwierig. – The (individual) expressions/phrases in this book are difficult.

Could I say Die Ausdrucksweise in diesem Buch ist schwierig instead? Is that more natural?

Yes, that’s also correct and quite natural:

Die Ausdrucksweise in diesem Buch ist schwierig.

Ausdrucksweise roughly means “way of expressing things / manner of expression / style of expression”.

Nuance:

Der Ausdruck in diesem Buch ist schwierig.
- Very common, a bit shorter and more general.
- Refers to the book’s phrasing / wording.
Die Ausdrucksweise in diesem Buch ist schwierig.
- Slightly more explicit about the style or manner of writing.
- A bit more formal / descriptive.

Both are fine; Der Ausdruck … is more frequent in everyday language.

Why can’t I just say Ausdruck in diesem Buch ist schwierig without der?

In German, countable singular nouns almost always need an article or some determiner (like der, ein, dieser, kein). You normally cannot leave it out the way you can in English with general statements.

So:

❌ Ausdruck in diesem Buch ist schwierig. – feels ungrammatical.
✅ Der Ausdruck in diesem Buch ist schwierig.
✅ Der sprachliche Ausdruck in diesem Buch ist schwierig.
✅ Dieser Ausdruck in diesem Buch ist schwierig. (if you mean one specific expression)

Omitting the article is common only in a few set contexts (headlines, telegram style, certain fixed phrases), but not in a normal sentence like this.