Spegillinn er viðkvæmur.

It is the definite article attached to the end of the noun.

• spegill = mirror
• spegillinn = the mirror

This is very common in Icelandic: instead of putting a separate word like the before the noun, Icelandic usually adds the article as an ending.

Here, spegillinn is masculine singular nominative definite.

Er is the present tense, 3rd person singular form of vera, which means to be.

So:

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

In Spegillinn er viðkvæmur, er simply means is.

Because Icelandic adjectives agree with the noun they describe in:

• gender
• number
• case

Spegillinn is:

• masculine
• singular
• nominative

So the adjective must also be masculine singular nominative, which gives viðkvæmur.

Compare:

• masculine: viðkvæmur
• feminine: viðkvæm
• neuter: viðkvæmt

So the ending -ur is there because spegillinn is a masculine noun.

Good question. This is because viðkvæmur here is a predicate adjective, not an adjective placed directly before the noun.

In other words:

• Spegillinn er viðkvæmur = The mirror is fragile
• viðkvæmi spegillinn = the fragile mirror

When the adjective comes after er and describes the subject, Icelandic normally uses the strong/predicate form: viðkvæmur.

When the adjective comes directly before a definite noun, it usually takes the weak form: viðkvæmi.

So:

• Spegillinn er viðkvæmur = correct
• viðkvæmi spegillinn = also correct, but it means the fragile mirror, not a full sentence by itself

Because the sentence uses the verb er = is.

Just like in English, when you use to be, the describing word usually comes after the verb:

• The mirror is fragile
• Spegillinn er viðkvæmur

If you want the adjective directly before the noun, you are no longer saying The mirror is fragile. You are saying something like the fragile mirror or a fragile mirror instead.

Compare:

• Spegillinn er viðkvæmur = The mirror is fragile
• viðkvæmur spegill = a fragile mirror
• viðkvæmi spegillinn = the fragile mirror

It can mean either, depending on context.

Common meanings include:

• fragile
• delicate
• sensitive
• vulnerable

With something like a mirror, the meaning is usually fragile/delicate.

With a person, topic, or situation, it could mean sensitive instead.

For example:

• viðkvæmur hlutur = a delicate / fragile object
• viðkvæm manneskja = a sensitive person
• viðkvæmt mál = a sensitive issue

So the exact English translation depends on what is being described.

Yes. Spegillinn is in the nominative case because it is the subject of the sentence.

The adjective viðkvæmur agrees with it, so it is also in the nominative masculine singular form.

So the structure is:

• Spegillinn = subject, nominative
• er = linking verb
• viðkvæmur = predicate adjective agreeing with the subject

This is a very common Icelandic pattern.

Grammatically, spegill without the article means a mirror, not the mirror.

So changing spegillinn to spegill changes the meaning:

• Spegillinn er viðkvæmur = The mirror is fragile
• Spegill er viðkvæmur = A mirror is fragile or a more generic statement

However, for a general statement, Icelandic often sounds more natural in the plural:

• Speglar eru viðkvæmir = Mirrors are fragile

So if you are talking about one specific mirror, spegillinn is the right form.

It would be:

• Speglarnir eru viðkvæmir. = The mirrors are fragile.

Here several things change:

• spegillinn → speglarnir
• er → eru
• viðkvæmur → viðkvæmir

That happens because the subject is now plural, so the verb and adjective must match it.

You could also say:

• Speglar eru viðkvæmir. = Mirrors are fragile.

A few things, yes:

• Icelandic stress is usually on the first syllable
• In viðkvæmur, ð is like the th in this
• In viðkvæmur, æ sounds like eye
• In spegillinn, the ll is not pronounced like a simple English l-l; it has a special Icelandic sound that many learners hear as tl- or dl-like

So the main things to watch are the special Icelandic consonants and the adjective’s vowel sounds.

English-style approximations can help at first, but it is best to listen to native pronunciation when possible.