Ég kem ekki á föstudag, því ég þarf að fara til tannlæknis.

Kem is the 1st person singular present tense of the verb að koma (to come).

So:

• að koma = to come
• ég kem = I come / I am coming / I will come

Icelandic verbs change form depending on the subject, just like English changes I am / he is, but usually more often.

In this sentence, the subject is ég (I), so the correct form is kem.

In Icelandic, the present tense is very often used to talk about the near future, especially when the context makes the time clear.

Here, the time expression á föstudag (on Friday) shows that this is about the future. So:

• Ég kem ekki á föstudag literally = I come not on Friday
• natural English meaning = I’m not coming on Friday

This is very common in Icelandic. English does something similar sometimes too, as in I’m going tomorrow.

Ekki means not, and in a normal main clause it usually comes after the finite verb.

So:

• Ég kem ekki = I am not coming
• Ég þarf ekki = I do not need
• Hann er ekki hér = He is not here

This word order can feel different from English, where not usually goes after an auxiliary verb like am, do, or will. In Icelandic, just remember that ekki commonly follows the conjugated verb.

Yes. In expressions of days, á is used much like English on.

So:

• á mánudag = on Monday
• á þriðjudag = on Tuesday
• á föstudag = on Friday

This is a very common pattern in Icelandic for days of the week.

Also, notice that föstudag is in a case form required after á in this time expression.

In Icelandic, days of the week are not capitalized, unless they begin a sentence.

So:

• mánudagur = Monday
• föstudagur = Friday

This is different from English, where weekday names are always capitalized.

Here því introduces the reason: because / since / for.

So the sentence means:

• Ég kem ekki á föstudag, því ég þarf að fara til tannlæknis.
• I’m not coming on Friday, because I need to go to the dentist.

A useful thing to know is that því often behaves a bit differently from af því að, which is another very common way to say because.

For example:

• Ég kem ekki, því ég er upptekinn.
• Ég kem ekki af því að ég er upptekinn.

Both can mean I’m not coming because I’m busy, but learners often notice that the word order can differ depending on which connector is used. In your sentence, því is followed by normal main-clause order: ég þarf.

The comma separates the main statement from the clause giving the reason.

• Ég kem ekki á föstudag = main statement
• því ég þarf að fara til tannlæknis = reason

Icelandic punctuation often uses commas in places that may feel slightly more frequent or different from English usage. In this sentence, the comma helps mark the explanatory clause.

Þarf is the present tense of að þurfa (to need / have to), and að fara means to go.

Together:

• ég þarf að fara = I need to go / I have to go

This is a very common Icelandic structure:

• þurfa að + infinitive

Examples:

• Ég þarf að vinna. = I need to work.
• Við þurfum að flýta okkur. = We need to hurry.
• Hún þarf að læra. = She needs to study.

So in your sentence, ég þarf að fara is simply I need to go.

After þurfa (need to), Icelandic normally uses að + infinitive.

So:

• þarf að fara = need to go
• þarf að vinna = need to work
• þarf að lesa = need to read

This is one of the most common infinitive patterns in Icelandic. English often uses to in the same way, so this part is fairly similar.

Because the preposition til requires the genitive case.

The dictionary form is:

• tannlæknir = dentist

But after til, it changes to the genitive singular:

• til tannlæknis = to the dentist

So this is a case issue, not a different word.

This is a very important pattern in Icelandic:

• til Reykjavíkur = to Reykjavík
• til mín = to me / to my place
• til læknis = to the doctor
• til tannlæknis = to the dentist

Icelandic often leaves out the definite article in expressions like go to the doctor, go to the dentist, go to school, especially when the meaning is general or idiomatic.

So:

• fara til tannlæknis = go to the dentist
• fara til læknis = go to the doctor

Even though English uses the, Icelandic often does not.

If you wanted a more specifically definite form in other contexts, Icelandic can add the article to the noun, but in this kind of everyday expression it is very natural to say simply til tannlæknis.

Literally, it may look more like go to dentist or go to a dentist, but idiomatically it usually means go to the dentist.

This is one of those places where you should translate the whole expression naturally, not word by word.

So the best English meaning here is:

• I need to go to the dentist

not

• I need to go to dentist

Yes, but it changes the nuance.

• á föstudag = on Friday
• á föstudaginn = on the Friday / this Friday / on Friday specifically

The version á föstudag is the more general and neutral way to say on Friday.

Adding the article (-inn) can make it sound more specific, depending on context.

In á föstudag, föstudag is in the accusative singular.

The basic form is:

• föstudagur = Friday

After á in this time expression, it appears as:

• á föstudag

This is a good example of how Icelandic nouns change form depending on grammatical case.

It is completely normal, natural everyday Icelandic.

Nothing in it sounds unusually formal, literary, or old-fashioned. A native speaker could easily say this in conversation, in a message, or in an email.

That makes it a very useful model sentence because it contains several common everyday patterns:

• Ég kem ekki... = I’m not coming...
• á föstudag = on Friday
• því... = because...
• ég þarf að... = I need to...
• fara til tannlæknis = go to the dentist