Learn Hardcore Dutch: In dit oefenboek zijn de zinnen kort, maar bij elke stap worden de opdrachten langzaam moeilijker. - In this exercise book the sentences are short, but with each step the exercises slowly become harder.

In your sentence, the first element is the place phrase In dit oefenboek (“in this exercise book”). Because that is put first for emphasis or context, the finite verb zijn must come next:

In dit oefenboek (first element)
zijn (verb in second position)
de zinnen kort (subject + rest)

You cannot say In dit oefenboek de zinnen zijn kort – that breaks the V2 rule and sounds wrong to Dutch speakers.

Could I also say De zinnen zijn kort in dit oefenboek? Does it mean the same?

Yes, De zinnen zijn kort in dit oefenboek is also correct and natural. The difference is mainly emphasis and flow:

In dit oefenboek zijn de zinnen kort
– Emphasis on the book: “In this book (as opposed to others), the sentences are short.”
De zinnen zijn kort in dit oefenboek
– Emphasis on the sentences: “The sentences are short in this book.”

Both are fine. Dutch word order is quite flexible as long as the finite verb stays in second position relative to the first element.

What exactly does bij mean in bij elke stap, and why not in elke stap or met elke stap?

In this context, bij elke stap means “at each step / at every stage”.

bij here expresses something like “in connection with / at the moment of”.
It’s very natural with words like stap (step), begin, start, controle (check):
- bij de eerste stap – at the first step
- bij controle – during the check / when checked

Alternatives:

in elke stap – sounds unnatural in this context; you normally make or take steps, not exist “in” them.
met elke stap – literally “with each step”. This can be used, but it often suggests a stronger feeling of accompaniment or “with every step you take”.
- Met elke stap werd hij moeër – With every step he took, he got more tired.

For describing the gradual increase in difficulty in a workbook, bij elke stap is the normal and idiomatic choice.

Why is worden used in worden de opdrachten langzaam moeilijker? I thought worden meant “to become” or was used for passive.

Worden has two main uses in Dutch:

“To become” (change of state) – that is what’s happening here.
- De opdrachten worden moeilijker – The exercises become / get more difficult.
To form the passive voice, together with a past participle:
- De opdrachten worden gemaakt – The exercises are being done.
- Het boek wordt gelezen – The book is being read.

In your sentence there is no past participle after worden, only an adjective moeilijker (more difficult). So here worden clearly means “to become”, not a passive.

Is worden here passive, or is it purely “to become”? How can I tell the difference?

Here it is purely “to become”, not passive. You can tell by the structure:

Passive: worden
- past participle
- Het huis wordt gebouwd – The house is being built.
- De opdrachten worden nagekeken – The exercises are checked.
“To become”: worden
- adjective (or comparative)
- Hij wordt boos – He becomes angry.
- Het wordt koud – It is getting cold.
- De opdrachten worden moeilijker – The exercises are becoming more difficult.

In your sentence we have worden + moeilijker (comparative adjective), so it’s the “to become” meaning.

Why is langzaam in the middle: worden de opdrachten langzaam moeilijker? Could I put it somewhere else?

Langzaam here is an adverb meaning “slowly / gradually”. In Dutch, adverbs like this often appear:

Between the finite verb and the adjective/complement
- De opdrachten worden langzaam moeilijker.

This is a very natural position.

Other possible placements (all correct, but with slightly different focus):

De opdrachten worden langzaam moeilijker.
– Neutral, very natural.
De opdrachten worden moeilijker, langzaam maar zeker.
– More like: “They get more difficult, slowly but surely.” (extra emphasis)
Langzaam worden de opdrachten moeilijker.
– Emphasis on gradually; also obeys V2: Langzaam (1st element), worden (2nd), then the rest.

The original placement is standard and sounds natural: worden … langzaam moeilijker.

How is moeilijker formed, and can I say meer moeilijk instead?

Moeilijker is the comparative form of the adjective moeilijk (difficult).

Formation:

Base adjective: moeilijk
Add -er: moeilijker = more difficult

For most one‑word adjectives in Dutch, you form the comparative with -er, not with meer:

groot → groter (big → bigger)
klein → kleiner (small → smaller)
snel → sneller (fast → faster)
moeilijk → moeilijker (difficult → more difficult)

So:

moeilijker is the normal, correct form.
meer moeilijk is grammatically possible but sounds very unnatural here; Dutch speakers will strongly prefer moeilijker.

Why do we have de zinnen and de opdrachten (with de and plural -en endings)? Could it also be without the article?

Both zin and opdracht are de‑words (common gender nouns):

de zin – the sentence
de opdracht – the exercise / task

Their regular plural is with -en:

de zinnen – the sentences
de opdrachten – the exercises

In your sentence these plurals refer to all the sentences and all the exercises in that specific book, so the definite article de is natural:

de zinnen – the (specific) sentences in this exercise book
de opdrachten – the (specific) exercises in this exercise book

You could in other contexts say zinnen or opdrachten without an article when speaking more generally (e.g. Zinnen moeten duidelijk zijn – “Sentences must be clear.”), but here we are clearly talking about the ones in this book, so de is appropriate.

What is the nuance of oefenboek compared to just boek? How do these compounds work in Dutch?

Oefenboek is a compound noun:

oefen (from oefening – exercise, practice)
boek (book)

Together: oefenboek = “workbook / exercise book”, i.e. a book specifically meant for doing exercises, not just reading.

Dutch forms many nouns this way:

werkboek – workbook (very close to oefenboek)
leerboek – textbook
kookboek – cookbook
woordenboek – dictionary (word‑book)

So:

boek = any book.
oefenboek = a type of book, specialized for exercises / practice.

Is maar exactly the same as English but here, and is the comma before maar necessary?

Yes, maar is the normal conjunction for “but” in Dutch, and it works in the same way here: it contrasts the short sentences with the increasing difficulty of the exercises.

About the comma:

Dutch often does put a comma before coordinating conjunctions like maar, especially when they join two full main clauses, as here:
- In dit oefenboek zijn de zinnen kort, maar bij elke stap worden de opdrachten langzaam moeilijker.
You will also see it without a comma in less formal writing:
- In dit oefenboek zijn de zinnen kort maar bij elke stap worden de opdrachten langzaam moeilijker.

The version with the comma is stylistically clearer and very common, especially in anything slightly formal or careful.

Could I say In dit oefenboek staan korte zinnen instead of In dit oefenboek zijn de zinnen kort?

Yes, you can, but the structure changes a bit:

In dit oefenboek zijn de zinnen kort.
– Literally: “In this exercise book the sentences are short.”
– Subject: de zinnen, predicate adjective: kort.
In dit oefenboek staan korte zinnen.
– Literally: “In this exercise book stand short sentences.”
– Subject: korte zinnen, verb: staan (used for printed text).

Dutch often uses staan (“to stand”) for text printed on a page or screen:

Er staat een fout in het boek. – There is a mistake in the book.
In de krant stond een artikel over jou. – There was an article about you in the newspaper.

So:

Both sentences are correct and natural.
Zijn de zinnen kort talks about the sentences and their property (they are short).
Staan korte zinnen emphasizes the presence of short sentences in the book (they appear/are printed there).

Can I move bij elke stap to the end and say De opdrachten worden langzaam moeilijker bij elke stap?

You can say De opdrachten worden langzaam moeilijker bij elke stap, and it is understandable, but it sounds a bit less smooth and slightly redundant to many native speakers.

More natural variants:

Bij elke stap worden de opdrachten langzaam moeilijker.
– This is the original word order (with bij elke stap at the beginning and verb‑second); very natural.
De opdrachten worden bij elke stap langzaam moeilijker.
– Also very natural: subject first, then verb, then bij elke stap.

Putting bij elke stap at the end is not wrong, but it can sound like an afterthought, as if you’re adding the condition at the last moment. For clear, neutral style, prefer one of the first two.