Drengen læser regelbogen.

Because -en is the definite singular ending for many common-gender nouns in Danish.

• dreng = boy / a boy
• drengen = the boy

Unlike English, Danish often adds the definite article to the end of the noun instead of using a separate word like the.

For the same reason: -en marks the definite singular.

• regelbog = rulebook / a rulebook
• regelbogen = the rulebook

So in this sentence, both nouns are definite:

• Drengen = the boy
• regelbogen = the rulebook

In Danish, the definite article is usually attached to the noun as a suffix.

So instead of:

• the boy
• the rulebook

Danish normally uses:

• drengen
• regelbogen

A separate definite word can appear in other structures, especially with adjectives, for example:

• den store dreng = the big boy

But in your sentence, no adjective is present, so the suffix is enough.

læser is the present tense of læse, which means to read.

• læse = to read
• læser = reads / is reading

The -er ending is very common for present-tense verbs in Danish.

Examples:

• jeg læser = I read / I am reading
• drengen læser = the boy reads / the boy is reading

It can mean both, depending on context.

So:

• Drengen læser regelbogen can mean The boy reads the rulebook
• or The boy is reading the rulebook

Danish present tense often covers both the simple present and the present continuous that English separates.

The word order is the normal subject–verb–object pattern:

• Drengen = subject
• læser = verb
• regelbogen = object

So literally:

• The boy
  • reads
    • the rulebook

This is the same basic order as in English.

Yes. Danish very often combines nouns into one word.

• regel = rule
• bog = book
• regelbog = rulebook
• regelbogen = the rulebook

This is extremely common in Danish, just as in German and some other Germanic languages.

The letter æ is a special Danish vowel. It does not sound exactly like a normal English a or e.

A rough guide:

• læser sounds approximately like LEH-ser, but with a more open vowel than English eh
• æ is somewhat similar to the vowel in English cat for many learners, though not identical

Also:

• læser has a soft danish r, not a strong English-style r

If you are learning pronunciation, it is best to listen to native audio, because Danish vowels can be tricky.

Both dreng and regelbog are common gender nouns, which is why their definite singular form takes -en.

In Danish, there are two grammatical genders:

• common gender
• neuter

For common gender nouns:

• indefinite article: en
• definite ending: often -en

So:

• en dreng = a boy
• drengen = the boy
• en regelbog = a rulebook
• regelbogen = the rulebook

It would be:

En dreng læser en regelbog.

That means: A boy reads a rulebook or A boy is reading a rulebook.

Compare:

• Drengen læser regelbogen. = The boy reads the rulebook.
• En dreng læser en regelbog. = A boy reads a rulebook.

Yes, if you mean the book rather than specifically the rulebook.

• bogen = the book
• regelbogen = the rulebook

So regelbogen is more specific. It tells you what kind of book it is.

One possible plural version is:

Drengene læser regelbogen. = The boys read the rulebook.

Here:

• dreng = boy
• drenge = boys
• drengene = the boys

If both nouns were plural, you could have something like:

• Drengene læser regelbøgerne. = The boys read the rulebooks.

Notice that plural forms change differently from singular ones, so they are worth learning separately.

Because modern Danish capitalization is similar to English:

• The first word of the sentence is capitalized
• Ordinary nouns are not capitalized

So:

• Drengen læser regelbogen.

This is different from German, where all nouns are capitalized. In Danish, drengen and regelbogen are lowercase unless they start the sentence.

Yes. In a yes/no question, Danish typically puts the verb before the subject:

• Læser drengen regelbogen? = Is the boy reading the rulebook? / Does the boy read the rulebook?

So the statement:

• Drengen læser regelbogen.

becomes the question:

• Læser drengen regelbogen?

This verb-second behavior is a very important feature of Danish word order.

Yes, that is a very good habit.

You should learn nouns like this:

• en dreng
• en regelbog

That helps you remember:

• the noun’s gender
• how to form the definite form more easily
• which article to use in new sentences

For Danish learners, memorizing nouns with their article is much more helpful than learning the bare noun alone.