Haugen er stor.

Because it is the first word in the sentence. In Norwegian, just like in English, the first word of a sentence is capitalized.

So here, Haugen does not necessarily mean it is a proper name. If the same word appeared later in the sentence as a common noun, it would normally be written haugen.

Example:

• Haugen er stor.
• Jeg ser haugen.

That said, Haugen can also be a surname or place name in other contexts, so context matters.

Because haugen is the definite singular form of haug.

• haug = a hill / a mound / a heap depending on context
• haugen = the hill / the mound / the heap

In Norwegian, definiteness is often added as an ending on the noun instead of using a separate word like the.

A simple pattern is:

• en haug = a hill / a heap
• haugen = the hill / the heap

So -en is doing the job that the does in English.

Because Norwegian usually expresses the by adding an ending to the noun.

Here:

• haugen already means the hill / the heap

So Norwegian does not need a separate article here.

This is very common in Norwegian:

• en bil = a car
• bilen = the car
• et hus = a house
• huset = the house

So in Haugen er stor, the definiteness is built into haugen.

Er means is.

It is the present tense of å være, which means to be.

Examples:

• Jeg er = I am
• du er = you are
• han er = he is
• haugen er stor = the hill is big

Because stor is the form used with a singular masculine or feminine noun in this kind of sentence.

Norwegian adjectives often change form depending on gender and number:

• stor = masculine/feminine singular
• stort = neuter singular
• store = plural

So:

• Haugen er stor. = singular, not neuter
• Huset er stort. = neuter singular
• Haugene er store. = plural

An important point: after er, the adjective agrees with gender and number, but not with definiteness.
That is why it is still stor, even though haugen is definite.

Compare:

• en stor haug = a big hill
• den store haugen = the big hill
• haugen er stor = the hill is big

The word order is:

subject + verb + adjective

So:

• Haugen = subject
• er = verb
• stor = adjective

This is very similar to English:

• The hill is big.

So this sentence is a nice example of a very straightforward Norwegian sentence pattern.

The dictionary form is haug.

When you look nouns up in a dictionary, you usually look for the indefinite singular form, not the definite form.

So if you want to find haugen, you should search for haug.

You may also see it listed with its article:

• en haug

That helps you know its gender and how it forms the definite singular:

• en haug → haugen

The exact pronunciation can vary by dialect, but in standard East Norwegian it is roughly like HOW-gen, with the first part sounding somewhat like how in English, though not exactly the same.

A rough guide:

• hau- ≈ how
• -gen ≈ gen with a soft, short ending

The g is pronounced here.

If you are learning Bokmål, remember that spelling stays the same, but pronunciation can differ quite a bit across Norway.

Yes, it could be in another context. Haugen is also a fairly common Norwegian surname and can appear in place names.

But in this sentence alone, you should not assume that just because it is capitalized. Since it is the first word of the sentence, capitalization is normal either way.

So:

• as a common noun: haugen = the hill / the heap
• as a name: Haugen could be a surname or place name

Context tells you which one is meant.