Trærne omringer hagen.

Because Norwegian often puts the definite article at the end of the noun instead of using a separate word like the.

• tre = tree
• trær = trees
• trærne = the trees

So -ne is the definite plural ending here.

Because tre has an irregular plural.

Its forms are:

• et tre = a tree
• treet = the tree
• trær = trees
• trærne = the trees

So this is just a noun form you need to learn as a set.

For the same reason: Norwegian usually marks definiteness with an ending.

• en hage = a garden
• hagen = the garden

Here, -n is the definite singular ending for this noun.

Because in Norwegian, the definite article is usually attached to the noun:

• trærne = the trees
• hagen = the garden

This is one of the big differences from English.
Instead of the + noun, Norwegian often uses noun + definite ending.

Omringer is the present tense of å omringe.

• å omringe = to surround
• omringer = surround / is surrounding

So Trærne omringer hagen means The trees surround the garden or, depending on context, The trees are surrounding the garden.

Because Norwegian verbs do not change according to the subject the way English verbs sometimes do.

In English:

• the tree surrounds
• the trees surround

In Norwegian, the verb form stays the same:

• Treet omringer hagen
• Trærne omringer hagen

So you do not need a different present-tense verb form for plural subjects.

Yes. This sentence uses normal subject–verb–object order:

• Trærne = subject
• omringer = verb
• hagen = object

So it works like:

The trees + surround + the garden

It mainly means to surround, encircle, or enclose from around the outside.

So Trærne omringer hagen gives the image that the garden has trees all around it.

Depending on context, English might translate it as:

• surround
• encircle
• ring

But surround is the most natural basic translation.

The æ sound is somewhat like the vowel in English cat for many speakers, though not exactly the same.

A rough pronunciation guide:

• trærne ≈ TRAIR-neh
• hagen ≈ HAH-gen
• omringer ≈ OM-ring-er

The exact pronunciation depends a bit on dialect, but the important thing for learners is recognizing that æ is a separate vowel sound and very common in Norwegian.

Yes, gender matters because it affects the indefinite article and some endings.

• tre is neuter: et tre
• hage is usually common gender (masculine in traditional grammar): en hage

That helps explain their forms:

• et tre → treet → trær → trærne
• en hage → hagen

So even though the sentence does not show et or en, knowing the gender helps you understand why the nouns change the way they do.