Frauðplastið er létt.

Because -ið is the definite article attached to the end of the noun. Icelandic often says the by adding it as an ending instead of using a separate word.

• frauðplast = foam plastic / Styrofoam
• frauðplastið = the foam plastic

So Frauðplastið er létt means The foam plastic is light.

It is neuter singular.

You can tell this from two things:

• the definite ending -ið, which is a common neuter singular definite ending
• the adjective létt, which is the form that agrees with a neuter singular noun here

In Icelandic, adjectives have to agree with the noun in gender, number, and case.

Because Icelandic adjectives change form to match the noun they describe.

The adjective léttur means light, but its form changes:

• masculine singular: léttur
• feminine singular: létt
• neuter singular: létt

Since frauðplastið is neuter singular, the correct form is létt.

Er means is.

It is the present tense, 3rd person singular form of the verb vera (to be).

So:

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

In this sentence, Frauðplastið er létt, er links the subject with the adjective, just like English is.

It is in the nominative case.

That is because it is the subject of the sentence: the thing being described as light.

In simple X is Y sentences, the subject is normally in the nominative:

• Frauðplastið = subject
• er = verb
• létt = predicate adjective

Because Icelandic usually expresses the by attaching it to the noun.

So instead of a separate word like English the, Icelandic often uses a suffix:

• hús = house
• húsið = the house

The same pattern appears here:

• frauðplast = foam plastic
• frauðplastið = the foam plastic

Yes, it can be, but the meaning changes slightly.

• Frauðplast er létt = Foam plastic is light / Foam plastic is a light material
• Frauðplastið er létt = The foam plastic is light

So without -ið, the noun is indefinite or more general. With -ið, it refers to something specific.

A rough guide is:

FROY-th-plas-tith er lyeht

A few useful notes:

• au in frauð- is pronounced roughly like oy in boy
• ð is a soft sound, somewhat like th in this
• -ið is often pronounced something like -ith
• é in létt sounds like ye in yes, but longer
• tt in Icelandic is not a normal English tt; it is pronounced with a kind of pre-aspirated sound, so létt does not sound exactly like English let

The exact pronunciation depends on accent and speaking style, but that is a helpful starting point.

It is an adjective.

Here it describes the noun frauðplastið, so it works like English light in The foam plastic is light.

It is not an adverb. An adverb would describe how something is done, like lightly in English.

No, this is a very normal Icelandic sentence pattern:

subject + verb + adjective

• Frauðplastið = subject
• er = verb
• létt = adjective

So it is directly comparable to English The foam plastic is light.

Yes, adjectives can go before nouns in Icelandic, but that would make a different structure.

For example, if you wanted to say the light foam plastic, the adjective would come before the noun and would still have to agree with it.

But in the sentence Frauðplastið er létt, létt is not directly attached to the noun inside a noun phrase. It comes after er and acts as a predicate adjective, meaning it tells you a quality of the subject.

So:

• létt frauðplast = light foam plastic
• frauðplastið er létt = the foam plastic is light

Because adjectives after vera usually just appear in the form that agrees with the noun in gender, number, and case. They do not take a separate definite article ending the way nouns do.

So the noun shows definiteness:

• frauðplastið = the foam plastic

And the adjective simply agrees with it:

• létt = light in the correct agreeing form

Usually it is being used more like a material noun, similar to plastic, wood, or metal in English.

That is why a sentence like Frauðplast er létt can sound natural as a general statement about the material.

But with frauðplastið, it can also refer to a specific piece, batch, or instance of that material, depending on context. Icelandic often lets context determine exactly how specific the noun is.