Lyklaborðið mitt er nýtt, en músin er gömul og virkar stundum illa.

In Icelandic, the definite article is usually attached to the end of the noun instead of being a separate word.

• lyklaborð = keyboard

• lyklaborðið = the keyboard

• mús = mouse
• músin = the mouse

So:

• lyklaborðið mitt = my keyboard
• músin = the mouse

This is one of the biggest differences from English.

The most neutral everyday way to say my keyboard in Icelandic is:

• lyklaborðið mitt

This pattern is very common:

• definite noun + possessive pronoun

So literally it looks like:

• the keyboard my

The version mitt lyklaborð is possible, but it is less neutral and can sound more contrastive, emphatic, or stylistically different. For a basic learner, lyklaborðið mitt is the safest pattern to remember.

Because Icelandic possessives agree with the noun they describe in gender, number, and case.

The noun lyklaborð is:

• singular
• neuter
• nominative here

So the correct form of my is mitt.

Compare:

• minn for a masculine singular noun
• mín for a feminine singular noun
• mitt for a neuter singular noun

For example:

• bíllinn minn = my car
• bókin mín = my book
• lyklaborðið mitt = my keyboard

Because nýr changes form to match the noun.

The subject lyklaborðið mitt is neuter singular, so the adjective must also be neuter singular:

• masculine: nýr
• feminine: ný
• neuter: nýtt

So:

• lyklaborðið mitt er nýtt = my keyboard is new

Even though lyklaborðið is definite, the adjective after er is still nýtt, not a definite form like the one you would use before a noun.

The adjective gamall = old is irregular, and it also has to agree with the noun.

The noun mús is feminine singular, so the adjective must be feminine singular too:

• masculine: gamall
• feminine: gömul
• neuter: gamalt

So:

• músin er gömul = the mouse is old

This is a very common adjective, so it is worth memorizing as a set.

By itself, músin means the mouse, not my mouse.

If you wanted to say my mouse, you would normally say:

• músin mín

So the sentence as written is specifically:

• my keyboard
• but the mouse

That may sound slightly uneven to an English speaker, but it is perfectly normal if the mouse is already understood from context.

Virkar is the present tense, 3rd person singular form of the verb virka.

Here are the relevant forms:

• infinitive: virka = to work, function
• present singular: virkar = works / functions

So:

• músin ... virkar stundum illa = the mouse ... sometimes works badly

In this sentence, virka means to work / function, not to seem.

Because illa is an adverb, and it modifies the verb virkar.

You need an adverb here because the sentence is describing how the mouse works:

• virkar illa = works badly

By contrast, illur / ill / illt are adjective forms and describe nouns, not verbs.

So:

• ill mús would mean a bad/sick mouse depending on context
• músin virkar illa means the mouse works badly

Also, not all Icelandic adverbs are formed with -lega. Some very common ones are irregular:

• góður → vel
• illur → illa

Because the subject músin is still understood.

The second part of the sentence is:

• músin er gömul og virkar stundum illa

The subject músin applies to both verbs:

• er
• virkar

English can do the same thing:

• The mouse is old and works badly sometimes.

You could repeat the pronoun in Icelandic:

• músin er gömul og hún virkar stundum illa

But it is not necessary here.

The main noun phrases are in the nominative because they are the subjects of the clause.

• lyklaborðið mitt = nominative
• músin = nominative

The predicate adjectives after er also match the subject:

• nýtt agrees with lyklaborðið
• gömul agrees with músin

So from a learner’s point of view, the important thing is:

• subject in nominative
• adjective agrees with the subject

The adverbs stundum and illa are not behaving like normal nouns or adjectives here, so case is not the main issue for them.

Because stundum is a time/frequency adverb and illa is a manner adverb.

The order here is very natural:

• virkar stundum illa
• works sometimes badly

In more natural English order, that becomes sometimes works badly or works badly sometimes.

This Icelandic order is common:

• verb
• frequency/time adverb
• manner adverb

So virkar stundum illa sounds very normal.

Yes, Icelandic word order is somewhat flexible, but not completely free.

This sentence uses a very neutral order:

• Lyklaborðið mitt er nýtt, en músin er gömul og virkar stundum illa.

Other arrangements are possible in different contexts, for example:

• Músin virkar stundum illa.
• Stundum virkar músin illa.

But if you move things around, Icelandic often requires other changes too, especially because it is a verb-second language in main clauses.

So for a learner, the safest takeaway is:

• virkar stundum illa is a normal, natural order
• you can move stundum, but you should learn those patterns carefully

Yes. Mús can mean both mouse the animal and mouse the computer device, just like in English.

In this sentence, the context makes it clear that it means the computer mouse, because it appears with lyklaborðið = the keyboard.

So:

• lyklaborðið mitt
  • músin strongly suggests computer equipment, not an animal.