Gildistíminn er langur.

Because Icelandic usually adds the definite article to the end of the noun instead of using a separate word like English the.

So:

• gildistími = validity period
• gildistíminn = the validity period

The ending -inn is the definite article here.

It is a compound noun plus the definite article:

• gildi = validity, value, or force/effect, depending on context
• tími = time or period
• gildistími = period of validity
• gildistíminn = the period of validity

A helpful thing to know is that Icelandic uses compound nouns very often, much more freely than English does.

That -s- is a very common linking element in Icelandic compounds. It often comes from a genitive form.

So you can think of:

• gildi
• gildis-
  • tími
• gildistími

This gives the sense of time of validity or validity period.

You do not always need to analyze every compound this way when speaking, but it is very useful for understanding how longer Icelandic words are built.

The -inn ending is the masculine singular nominative definite article attached to the noun.

The base noun is gildistími, and when the article is added, you get:

• gildistími
  • -inn → gildistíminn

So the double n is just part of how the definite form is spelled in this pattern. It is not a separate extra ending with a separate meaning.

It is:

• masculine
• singular
• nominative

Why?

• tími is a masculine noun, and compounds normally take the gender of the final element
• it is singular because it refers to one validity period
• it is nominative because it is the subject of the sentence

This matters because the adjective has to agree with it.

Because langur agrees with gildistíminn in:

• gender: masculine
• number: singular
• case: nominative

So the correct form here is langur.

Also, this is a predicate adjective after er. In Icelandic, predicate adjectives are normally in the strong form, even when the noun is definite.

That is why you get:

• Gildistíminn er langur

not:

• Gildistíminn er langi

Very roughly:

• langur = masculine singular nominative
• löng = feminine singular nominative
• langt = neuter singular nominative

Er is the present tense, third-person singular form of vera, which means to be.

So it works like English is:

• ég er = I am
• þú ert = you are
• hann/hún/það er = he/she/it is

In this sentence, er links the subject gildistíminn with the description langur.

It can mean either, depending on the noun it describes.

With a word like gildistími, it refers to duration, not physical size. So it means that the validity period lasts a long time.

This is similar to English, where long can describe both:

• a long road
• a long meeting

But for people, English tall is not usually langur in Icelandic. For a person being tall, Icelandic normally uses hár.

Yes, but the meaning changes.

• Gildistíminn er langur = the validity period is long
• Gildistími er langur = a validity period is long or validity period is long in a more general sense

The version with -inn is definite and points to a specific validity period. The version without it is indefinite or more general.

A yes/no question is made by putting the finite verb first:

• Er gildistíminn langur?

This means Is the validity period long?

So:

• statement: Gildistíminn er langur.
• question: Er gildistíminn langur?

That verb-first pattern is very common in Icelandic yes/no questions.

A few useful pronunciation points:

• Stress normally falls on the first syllable of each word
• í is pronounced like ee in see
• r is usually a trilled or tapped r
• langur is not pronounced like English langer; keep it distinctly Icelandic

A rough stress guide would be:

• GIL-di-stí-minn er LANG-ur

You do not need perfect pronunciation right away, but noticing first-syllable stress will help a lot.