Eerst vond ik het boek ingewikkeld, maar nu begrijp ik het beter.

This is because Dutch main clauses follow the V2 rule: the conjugated verb must be in the second position.

So if Eerst is placed first, the verb has to come next:

• Eerst vond ik het boek ingewikkeld.

Structure:

• Eerst = first position
• vond = second position
• ik = subject, after the verb

If the subject comes first, then you get:

• Ik vond het boek eerst ingewikkeld.

That is possible, but it changes the emphasis. The original sentence highlights the time contrast: at first ... but now ...

Not literally. In this sentence, vinden means to think or to consider.

A very common Dutch pattern is:

• iets + adjective + vinden

So:

• ik vond het boek ingewikkeld

means:

• I thought the book was complicated
• I found the book complicated

This is just like English to find something interesting / difficult / boring.

Other examples:

• Ik vind dit leuk. = I like this / I find this nice.
• Zij vonden de film saai. = They found the film boring.

Vond is the past tense of vinden.

Here are the main forms:

• ik vind = I find / I think
• ik vond = I found / I thought
• gevonden = found

The speaker is talking about an earlier opinion, so the past tense is used:

• Eerst vond ik het boek ingewikkeld = earlier, that was my impression

Then the sentence switches to the present:

• nu begrijp ik het beter = now, at this moment, I understand it better

So the tense contrast matches the time contrast:

• earlier -> past tense
• now -> present tense

Because boek is a neuter noun in Dutch, so its definite article is het.

• de tafel
• het boek

This is something you usually have to learn with each noun.

Because boek is a het-word, the pronoun referring back to it is also het:

• het boek
• ik begrijp het beter

So the second het means it, referring to the book.

Because ingewikkeld is not directly in front of the noun here. It is being used as a predicative adjective (an adjective that describes something through the verb).

Compare:

• het ingewikkelde boek = the complicated book
  -> adjective directly before the noun, so it gets -e

but:

• het boek is ingewikkeld = the book is complicated
• ik vond het boek ingewikkeld = I found the book complicated
  -> adjective is not directly before the noun, so no -e

This is a very common difference in Dutch.

Because the structure is:

• subject + vinden + object + adjective

So:

• ik = subject
• vond = verb
• het boek = object
• ingewikkeld = description of that object

This is normal in Dutch with verbs like vinden:

• Ik vind deze oefening moeilijk.
• Wij vonden de les interessant.
• Zij vindt hem aardig.

English does something similar:

• I found the book complicated
• They found the lesson interesting

Because the second part is still a main clause, and main clauses in Dutch still follow the V2 rule.

After maar, the clause begins with nu, so the verb comes next:

• maar nu begrijp ik het beter

Structure:

• maar = coordinating conjunction
• nu = first position in the new clause
• begrijp = conjugated verb in second position
• ik = subject

So maar nu ik begrijp het beter is not correct as a main clause.

However, this is correct:

• Maar ik begrijp het nu beter.

That just puts the emphasis somewhere else. The original sentence emphasizes the contrast in time: then vs now.

The second het is the pronoun it, referring back to het boek.

So:

• het boek -> het

This is very common in Dutch:

• Ik lees het boek. Ik begrijp het nu beter.

Because boek is a singular het-word, the pronoun is het.

Because beter means better, and the sentence is making a comparison with the past.

• Eerst = at first
• nu = now

So the idea is:

• before, understanding was weaker
• now, understanding is stronger

That is why beter is natural:

• nu begrijp ik het beter = now I understand it better

Compare:

• Nu begrijp ik het goed. = Now I understand it well.
• Nu begrijp ik het beter. = Now I understand it better.

The first says the understanding is good now.
The second explicitly compares now with before.

Here it means at first.

In Dutch, eerst can have a few related meanings depending on context:

• eerst = first / at first / first of all / before

In this sentence, because it contrasts with maar nu, it clearly means at first:

• Eerst vond ik het boek ingewikkeld, maar nu ...

Other natural alternatives would be:

• In het begin vond ik het boek ingewikkeld...
• Aanvankelijk vond ik het boek ingewikkeld...

Nuance:

• eerst is common and natural in everyday Dutch
• aanvankelijk is more formal
• in het begin is also very common and clear

Because maar is joining two full main clauses:

• Eerst vond ik het boek ingewikkeld
• maar nu begrijp ik het beter

In Dutch, it is normal to place a comma before maar when it connects two independent clauses, especially when each clause has its own subject and verb.

So the comma helps show the contrast clearly:

• first state
• then contrasting current state

It works much like English punctuation in:

• At first I found the book complicated, but now I understand it better.