Prisgarantien gjelder i seks måneder.

It is a compound noun:

• pris = price
• garanti = guarantee
• -en = the definite ending, meaning the

So prisgarantien literally means the price guarantee.

Norwegian very often combines nouns into one longer word, so this is completely normal.

Because the noun is in the definite singular form.

The base word is:

• en prisgaranti = a price guarantee

The definite form is:

• prisgarantien = the price guarantee

For many common-gender nouns in Bokmål, the definite singular ending is -en.

In Norwegian, the is usually added to the end of the noun instead of appearing as a separate word.

So:

• en garanti = a guarantee
• garantien = the guarantee

That same pattern appears here:

• en prisgaranti = a price guarantee
• prisgarantien = the price guarantee

English uses a separate article; Norwegian often uses a suffix.

Here gjelder means something like:

• applies
• is valid
• is in effect

So in this sentence, it means the price guarantee is valid for that period.

The verb å gjelde can have slightly different meanings depending on context. It can mean:

• to apply
• to concern
• to be valid

In this sentence, to be valid or to apply is the best fit.

The dictionary form is å gjelde.

gjelder is the present tense form:

• å gjelde = to apply / to be valid
• gjelder = applies / is valid

So if you look this word up in a dictionary, you should search for gjelde, not gjelder.

Because Norwegian often uses i to express duration.

So:

• i seks måneder = for six months

Even though i often means in, here it does not mean in six months in the English sense of six months from now.

A useful contrast:

• i seks måneder = for six months / during a six-month period
• om seks måneder = in six months / six months from now

That is a very important distinction.

Because after numbers greater than one, Norwegian normally uses the indefinite plural.

So:

• én måned = one month
• to måneder = two months
• seks måneder = six months

Why not the others?

• måned would be singular, so it would not match seks
• månedene means the months, which is not what the sentence is saying

So seks måneder is the normal and correct form.

Yes. Norwegian normally writes compound nouns as one word.

So:

• prisgaranti is correct
• pris garanti is not correct

This is one of the biggest differences from English. English often uses separate words where Norwegian prefers a compound.

Other similar examples:

• bilforsikring = car insurance
• prisforskjell = price difference
• kundeservice = customer service

The order is:

• Prisgarantien = subject
• gjelder = verb
• i seks måneder = time expression

So the structure is basically:

• Subject + Verb + Time phrase

This is a very normal Norwegian main-clause pattern.

Also remember that Norwegian is a verb-second language in main clauses. That means the finite verb normally comes in the second position.

Here, the subject comes first, so the verb comes right after it:

• Prisgarantien gjelder i seks måneder.

Yes, that would also be understandable and natural.

There is a small difference in feel:

• Prisgarantien gjelder i seks måneder = the price guarantee applies for six months
• Prisgarantien er gyldig i seks måneder = the price guarantee is valid for six months

Both work, but gjelder is very common for things like:

• guarantees
• rules
• offers
• agreements
• conditions

So the original sentence sounds very idiomatic.

A useful rough guide is YEL-der.

A few helpful points:

• the gj at the beginning is often pronounced roughly like y in yes
• e is short here
• the word has two syllables: gjel-der

So an English-friendly approximation is:

• YEL-der

The exact pronunciation varies somewhat by dialect, but that approximation is good enough for a learner.