Sidebørsten ligger ved siden av hovedbørsten.

Because they are in the definite singular form.

In Norwegian, instead of usually putting a separate word like the before the noun, you often add the definiteness to the end of the noun:

• en sidebørste = a side brush
• sidebørsten = the side brush

And:

• en hovedbørste = a main brush
• hovedbørsten = the main brush

So -en here works like English the.

Ligger is the present tense of å ligge, which often means to lie or to be lying/located.

In Norwegian, å ligge is very commonly used for the position or location of things, especially when they are physically resting somewhere.

So:

• Sidebørsten er ved siden av hovedbørsten would sound more neutral and less physical.
• Sidebørsten ligger ved siden av hovedbørsten emphasizes that the side brush is physically situated there.

English often just uses is, but Norwegian frequently chooses a more specific verb such as:

• ligge = lie, be located horizontally / be situated
• stå = stand
• sitte = sit

This is very natural in Norwegian.

Ved siden av means beside, next to, or at the side of.

It is a fixed expression made up of:

• ved = by / at
• siden = the side
• av = of

Literally, it is something like by the side of.

So:

• ved siden av bordet = beside the table
• ved siden av hovedbørsten = beside the main brush

You should learn ved siden av as a chunk, because Norwegian uses this full expression very often.

Because Norwegian usually puts the at the end of the noun instead of using a separate article.

Compare:

• English: the brush
• Norwegian: børsten

This is one of the biggest structural differences from English.

However, Norwegian can also use a separate determiner together with a suffixed definite noun in some situations, for example:

• den store børsten = the big brush

But in your sentence, no extra word is needed, because the nouns are simple definite nouns:

• sidebørsten
• hovedbørsten

Yes. Norwegian very often forms compound nouns by joining words together.

Here:

• hoved = main
• børste = brush

So:

• hovedbørste = main brush
• hovedbørsten = the main brush

Likewise:

• side = side
• børste = brush
• sidebørste = side brush
• sidebørsten = the side brush

This is extremely common in Norwegian. Where English often writes two words, Norwegian often uses one compound word.

Yes, depending on the variety of Norwegian and the speaker’s style.

In Bokmål, many nouns can be either masculine or feminine. Børste is often treated as feminine by many speakers, but masculine forms are also possible in Bokmål.

So you may see:

• en børste / børsten
• ei børste / børsta

That means this sentence could also appear in a form like:

• Sidebørsta ligger ved siden av hovedbørsta.

That would still be correct in many forms of Bokmål.
The version with -en is very common and safe to use.

Because Norwegian is a V2 language, which means the finite verb normally comes in the second main slot of a main clause.

In your sentence:

• Sidebørsten = first element
• ligger = second element
• ved siden av hovedbørsten = rest of the sentence

This is the standard Norwegian pattern.

You can change what comes first, but then the verb still stays second:

• Ved siden av hovedbørsten ligger sidebørsten.

That is also correct. It just gives a different emphasis.

This verb-second rule is very important in Norwegian.

Sometimes, but ved siden av is the standard and most natural expression for next to / beside in many contexts.

• ved siden av = the usual expression for physical location beside something
• på siden av can appear in some contexts, but it often sounds less standard here or can suggest slightly different nuance depending on the sentence

For this sentence, ved siden av is the best choice.

So a learner should remember:

• X ligger ved siden av Y = X is next to Y

The letter ø is a vowel that does not exist in standard English, so it can feel tricky at first.

A rough guide:

• Put your lips in a rounded shape, almost like for o
• But try to make a vowel somewhat closer to e or uh in the front of the mouth

It is similar to vowels found in some varieties of French or German, but not in most English accents.

So:

• børste
• sidebørsten
• hovedbørsten

all contain that rounded vowel.

If you cannot produce it perfectly at first, that is normal. It is better to aim for a clear, consistent vowel and improve with listening practice.

Not always. Norwegian uses ligge more broadly than English uses lie.

It can describe:

• something physically lying somewhere
• something located somewhere
• something placed somewhere

So even if the object is not dramatically lying flat, ligger may still be the natural verb if the item is resting in a place.

For objects, Norwegian often chooses between:

• ligger if something is lying/resting
• står if something stands upright
• sitter if something is attached, seated, or positioned in a certain way

In technical or descriptive contexts, this kind of choice is very common.

Yes. You could say:

• Hovedbørsten ligger ved siden av sidebørsten.

That simply changes which object is presented as the starting point of the sentence.

Your original sentence focuses on the side brush and says where it is in relation to the main brush.

The alternative focuses on the main brush and says where it is in relation to the side brush.

Both are grammatically correct. The choice depends on what you want to talk about first.

This sentence is standard Bokmål.

A Nynorsk version would likely look different, for example with forms such as:

• ligg instead of ligger
• possibly other noun choices depending on context

But the given sentence is clearly natural Bokmål:

• Sidebørsten ligger ved siden av hovedbørsten.

If you are learning Norwegian from most textbooks, apps, and courses, you are very likely learning Bokmål, so this version is the one to focus on.