Im Wörterheft markiere ich mit dem Textmarker nur die schwierigsten Wörter.

Because German often puts a time, place, or other context element first.

Here, Im Wörterheft means in the vocabulary notebook, and it takes the first position in the sentence. In a normal main clause, the conjugated verb must still stay in second position, so the next word is markiere:

• Im Wörterheft markiere ich ...

If you started with Ich, that would also be correct:

• Ich markiere im Wörterheft nur die schwierigsten Wörter.

The difference is mostly about emphasis. Starting with Im Wörterheft puts more focus on where this happens.

Im is a contraction of in dem.

• in dem Wörterheft → im Wörterheft

German uses this contraction very often.

Here, dem is dative, because the phrase describes a location rather than movement:

• im Wörterheft = in the notebook
• not movement into the notebook, but location inside it

So this is the same idea as:

• Ich schreibe im Heft.
• Das Wort steht im Buch.

Because German main clauses follow the verb-second rule.

In a statement, the conjugated verb must be the second element. Since Im Wörterheft is in first position, markiere must come next, and ich comes after that:

• Im Wörterheft = first element
• markiere = second element
• ich = subject, after the verb

Compare:

• Ich markiere im Wörterheft ...
• Im Wörterheft markiere ich ...

Both are correct. The verb stays in second position in both sentences.

It is dative, because the preposition mit always takes the dative case.

So:

• mit dem Textmarker = with the highlighter

Examples:

• mit dem Stift
• mit der Schere
• mit den Freunden

Here, dem Textmarker expresses the instrument: the thing used to do the marking.

Textmarker is the usual German word for a highlighter.

Literally, it looks like text marker, which is why it may seem strange to an English speaker. But in normal German, Textmarker is the standard everyday word for the pen used to highlight text.

So:

• mit dem Textmarker markieren = to highlight with a highlighter

This phrase means the most difficult words, and it contains a superlative adjective.

Breakdown:

• die = the (plural, here accusative plural)
• schwierigsten = most difficult
• Wörter = words

The adjective comes from schwierig:

• schwierig = difficult
• schwieriger = more difficult
• am schwierigsten = most difficult

Before a noun, German uses the attributive superlative form:

• die schwierigsten Wörter

Because there is already a definite article (die), the adjective takes the weak ending -en:

• die schwierigsten Wörter

Compare:

• schwierige Wörter = difficult words
• die schwierigen Wörter = the difficult words
• die schwierigsten Wörter = the most difficult words

German has two plural forms of Wort:

• Wörter = individual words, like vocabulary items
• Worte = words in the sense of utterances, sayings, or connected speech

In this sentence, we are talking about vocabulary items in a notebook, so Wörter is the natural choice.

Examples:

• Ich lerne neue Wörter. = vocabulary words
• Seine Worte klangen traurig. = his words/speech sounded sad

Yes. German very often builds compound nouns by joining words together.

• Wörter = words
• Heft = notebook/exercise book
• Wörterheft = vocabulary notebook / word notebook

This is completely normal in German. English often uses separate words where German prefers one compound word.

Other examples:

• Hausaufgabe
• Schreibtisch
• Lehrbuch

Also remember: German nouns are capitalized, so Wörterheft and Textmarker both start with capital letters.

Because nur usually goes near the part it is limiting.

Here:

• nur die schwierigsten Wörter

means that the speaker marks only the most difficult words, not all words.

So nur specifically limits the noun phrase that follows it.

If you move nur, the emphasis can shift. For example:

• Ich markiere nur die schwierigsten Wörter.
  = I mark only the most difficult words.

• Nur ich markiere die schwierigsten Wörter.
  = Only I mark the most difficult words.

So word placement matters.

It is accusative plural, because it is the direct object of markiere.

Ask:

• What do I mark?
• die schwierigsten Wörter

That is the thing being marked, so it is the direct object.

In plural, the definite article is:

• nominative plural: die
• accusative plural: die

So the article looks the same in both cases, but here the function is accusative because it is the object of the verb.

Yes. German word order is flexible, as long as the verb-second rule is respected in a main clause.

These are all possible:

• Im Wörterheft markiere ich mit dem Textmarker nur die schwierigsten Wörter.
• Ich markiere im Wörterheft mit dem Textmarker nur die schwierigsten Wörter.
• Mit dem Textmarker markiere ich im Wörterheft nur die schwierigsten Wörter.

All are grammatical. The main difference is focus:

• Im Wörterheft ... emphasizes the place
• Mit dem Textmarker ... emphasizes the tool
• Ich ... is the most neutral start

Because German normally does not use a helper verb like English do in ordinary statements.

English can say:

• I do mark... for emphasis

German usually just uses the main verb itself:

• Ich markiere ...

If you want emphasis in German, you usually change word order, use stress, or add words like doch, wirklich, or tatsächlich, depending on context.

So the plain German sentence simply uses:

• markiere = mark

In many contexts, yes. markieren is a common and natural verb for marking or highlighting something.

With Textmarker, it clearly means highlighting text.

You may also hear:

• hervorheben = to emphasize / highlight
• unterstreichen = to underline

But in this sentence, markieren works very naturally because the tool is specifically a Textmarker.