Glerumbúðirnar eru hreinar.

Because Icelandic uses compounds very freely.

glerumbúðirnar can be broken down like this:

• gler = glass
• umbúðir = packaging, wrappings, containers
• -nar = the definite article ending here

So Icelandic packs the whole idea into one compound noun instead of writing separate words the way English often does.

It is attached to the noun.

In Icelandic, the is very often a suffix rather than a separate word. In glerumbúðirnar, the ending -nar means the.

So:

• glerumbúðir = glass packaging / glass containers
• glerumbúðirnar = the glass packaging / the glass containers

Because the subject is plural.

• er = is
• eru = are

Here, glerumbúðirnar is grammatically plural, so Icelandic uses eru.

That is true even if English might sometimes use a singular-looking translation such as the packaging is clean. Icelandic still treats umbúðir as plural in this sentence.

Yes, umbúðir is normally a plural word in Icelandic when talking about packaging, wrappings, and similar things.

There is a singular form, umbúð, but in everyday use the plural is very common. This means Icelandic may use plural grammar where English uses a singular or uncountable noun.

So a learner should get used to things like:

• Umbúðirnar eru... = the packaging is / the packages are...
• plural verb
• plural adjective

Because Icelandic adjectives must agree with the noun they describe in number, gender, and usually case.

Here the subject is:

• plural
• feminine
• nominative

So the adjective also appears in the feminine plural nominative form:

• hreinn = clean, base/dictionary form
• hreinar = feminine plural nominative

That is why the sentence has hreinar.

The noun is in the nominative case because it is the subject of the sentence.

With the verb vera = to be, a predicate adjective normally agrees with the subject, so hreinar is nominative too.

So:

• glerumbúðirnar = nominative plural feminine
• hreinar = nominative plural feminine

This kind of matching is very common in Icelandic.

In this sentence, the adjective is being used predicatively, not attributively.

Compare:

• Glerumbúðirnar eru hreinar. = the glass packaging/containers are clean
• hreinar glerumbúðir = clean glass packaging/containers

So in your sentence, hreinar comes after eru because it is part of the statement about the subject.

This also affects the adjective form. Predicate adjectives after vera use the strong form here:

• eru hreinar

But before a definite noun, you would normally use a weak adjective form:

• hreinu glerumbúðirnar = the clean glass packaging/containers

In this compound, gler is just the first element of the compound and stays as it is.

The main noun is umbúðir, and that is the part that controls the grammar:

• plural
• feminine
• definite

So the agreement in eru hreinar is based on umbúðir, not on gler.

This is a useful rule for compounds in Icelandic: the last part is usually the grammatical head.

This is the normal, neutral word order for a simple statement:

• Glerumbúðirnar = subject
• eru = verb
• hreinar = predicate adjective

Icelandic does allow other word orders, but the verb usually stays in the second position in main clauses.

For example:

• Núna eru glerumbúðirnar hreinar. = Now the glass packaging/containers are clean.

So your sentence is the most straightforward, standard way to say it.

A rough learner-friendly guide is:

GLER-um-boo-thir-nar EH-ru HRAY-nar

A few helpful points:

• Stress usually falls on the first syllable of each word.
• ú sounds roughly like oo in food.
• ð sounds roughly like th in this.
• ei in hreinar sounds roughly like ay in day.
• The hr at the start of hreinar is not quite like English, but saying hr is a good beginner approximation.

If you want, you can think of the rhythm as:

GLER-um-bú-ðir-nar | ER-u | HREI-nar