Vi behöver längre gardiner eftersom fönstren är så höga.

Because längre is the comparative form of lång (long).

• lång = long
• längre = longer
• längst = longest

So längre gardiner means longer curtains.

This is not regular from an English point of view, because the vowel changes:

• lång → längre

Also, when a comparative adjective comes before a noun, it does not change for gender or number:

• ett längre rep
• längre gardiner

So längre works the same whether the noun is singular or plural.

Gardiner is the indefinite plural form: curtains.

• en gardin = a curtain
• gardiner = curtains
• gardinerna = the curtains

In this sentence, the speaker means they need some longer curtains, not necessarily a specific already-identified set of curtains. That is why Swedish uses the indefinite plural gardiner.

Fönstren is the definite plural of fönster.

The noun fönster is an ett word:

• ett fönster = a window
• fönster = windows
• fönstren = the windows

This noun is a little unusual because the indefinite singular and indefinite plural look the same:

• ett fönster
• två fönster

The definite plural adds -en:

• fönstren

So fönstren är så höga means the windows are so high.

Because höga agrees with fönstren, which is plural.

After är, Swedish adjectives still agree with the noun:

• fönstret är högt = the window is high
• fönstren är höga = the windows are high

Compare the forms of hög:

• hög = common gender singular
• högt = neuter singular
• höga = plural

Since fönstren is plural, the correct form is höga.

Because the two adjectives describe different kinds of measurement.

• hög is used for something that has great vertical height: a window, a building, a wall
• lång is used for something that has great length: a rope, a dress, curtains

So the windows are described as höga because they are high/tall, and the curtains must be längre because they need more length to cover that height.

This is very natural in Swedish:

• höga fönster
• långa gardiner

Eftersom means because or since.

It introduces a reason:

• Vi behöver längre gardiner eftersom fönstren är så höga.

So the second part explains the reason for the first part.

It is a common and neutral word in both spoken and written Swedish.

Yes. Eftersom introduces a subordinate clause, and subordinate clauses normally keep the order subject + verb.

So you get:

• eftersom fönstren är så höga

not:

• eftersom är fönstren så höga

That is different from some main-clause patterns in Swedish, where the verb often comes in second position.

A useful thing to remember:

• after eftersom, use normal clause order: subject + verb

Here så means so in the sense of to such a degree.

• så höga = so high

It strengthens the adjective höga.

In this sentence, så helps express the idea that the windows are high enough to create the need for longer curtains.

You will often see så before adjectives:

• så stor = so big
• så dyr = so expensive
• så höga = so high

Yes. Behöva can take a direct object, just like need in English.

• Vi behöver längre gardiner. = We need longer curtains.

You do not need another verb after it here.

You could also say something like:

• Vi behöver ha längre gardiner

but that is more explicit and usually less natural in a simple sentence like this. The shorter version with just the noun is the normal choice.

Yes, but then the main clause changes word order.

You can say:

• Eftersom fönstren är så höga behöver vi längre gardiner.

Notice what happens in the main clause:

• behöver vi not
• vi behöver

That is because when a subordinate clause comes first, Swedish main-clause word order puts the finite verb before the subject.

So:

• Vi behöver längre gardiner eftersom fönstren är så höga.
• Eftersom fönstren är så höga behöver vi längre gardiner.

Both are correct.

Yes. Därför att can also mean because.

So this would also work:

• Vi behöver längre gardiner därför att fönstren är så höga.

In many situations, eftersom and därför att are very close in meaning.

Be a little careful with för att, though. It can sometimes mean because in speech, but very often it means in order to. For learners, eftersom is usually the safest and clearest choice when you want to say because.