Efnið er mjúkt.

Er is the present-tense singular form of vera, the Icelandic verb to be.
So grammatically, it works just like English is.

• vera = to be
• er = is

The base noun is efni. Icelandic usually adds the definite article to the end of the noun instead of using a separate word like English the.

So:

• efni = material, substance, subject matter, etc.
• efnið = the material / the substance

In this form, the noun is neuter singular nominative, and the definite ending appears as -ð, producing the written form efnið after a noun ending in -i.

Because Icelandic normally uses a suffixed definite article. That means the is attached to the noun itself.

So instead of something like English the material, Icelandic says:

• efnið

This is one of the first big differences English speakers notice.

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

The noun efnið is neuter singular, so the adjective has to be in the neuter singular form too:

• dictionary form: mjúkur = soft
• neuter singular: mjúkt

So mjúkt matches efnið.

Yes. Icelandic adjectives still agree with the noun even when they are used after the verb, as a predicate adjective.

So in:

• Efnið er mjúkt

the adjective mjúkt still agrees with efnið.

This is similar to English in meaning, but Icelandic shows the agreement more clearly in the adjective ending.

Here they are nominative singular neuter.

Why?

• efnið is the subject, so it is in the nominative
• after vera (to be), the describing adjective is also normally in the nominative and agrees with the subject

So both parts line up grammatically:

• efnið = nominative singular neuter
• mjúkt = nominative singular neuter

A rough English-friendly guide would be:

• Efnið ≈ EP-nith
• er ≈ er with a tapped or rolled r
• mjúkt ≈ myookt, with j sounding like English y

A few useful sound notes:

• ð is like th in this
• j is like y in yes
• ú is like oo in food
• the final kt in mjúkt is sharper than in English

English-style approximations are never perfect, but this will get you close.

Yes. Efni is a very common Icelandic word with several related meanings, depending on context. It can mean things like:

• material
• substance
• matter
• content
• subject matter
• topic

In Efnið er mjúkt, the adjective mjúkt strongly suggests a physical substance or material.

You would put the adjective directly before the noun:

• mjúkt efni = soft material

That is different from:

• Efnið er mjúkt = the material is soft

So:

• mjúkt efni = adjective + noun
• efnið er mjúkt = noun + is
  • adjective

Both use the same neuter singular adjective form mjúkt, because efni is a neuter noun.