Mér er illt í tönninni aftur.

Because Icelandic often uses an impersonal construction for physical sensations.

• mér is the dative form of ég (I / me)
• er = is

So mér er illt literally means something like to me, it is painful, not I am painful.

This pattern is very common with feelings and bodily discomfort:

• Mér er kalt = I am cold
• Mér er illt = I am in pain / It hurts

So in this sentence, Icelandic expresses the pain as a state affecting you, rather than describing you directly with a normal subject.

Here illt means painful / sore / hurting.

It comes from illur, which often means bad in other contexts, but in this structure mér er illt it specifically means it hurts or I’m in pain.

So:

• Mér er illt = I’m hurting / I’m in pain
• Mér er illt í bakinu = My back hurts
• Mér er illt í höfðinu = My head hurts

It is a very common word in sentences about physical pain.

Because in this expression, Icelandic uses the neuter singular form: illt.

The pattern is basically:

• mér er + adjective in neuter singular + í + body part

So you get:

• Mér er kalt
• Mér er heitt
• Mér er illt

This is not agreeing with mér. It is part of an impersonal structure where the adjective appears in the neuter singular form.

The phrase vera illt í einhverju means to have pain in something / for something to hurt.

So:

• í here means in
• tönninni means the tooth

Literally:

• Mér er illt í tönninni = It is painful to me in the tooth

In natural English, that becomes:

• My tooth hurts
• I have a toothache

This í + body part pattern is very common:

• Mér er illt í höfðinu = My head hurts
• Mér er illt í bakinu = My back hurts
• Mér er illt í fætinum = My leg hurts

Tönninni is in the dative singular.

That is because the preposition í takes the dative when it expresses location rather than movement.

Here there is no movement into the tooth; it means pain is located there. So Icelandic uses:

• í + dative

The noun is:

• tönn = tooth
• tönninni = the tooth (dative singular with the definite article)

So this is not an arbitrary form; it is required by the preposition í in this meaning.

That ending includes the definite article and the case ending.

Icelandic usually puts the onto the end of the noun instead of writing it as a separate word.

So:

• tönn = tooth
• tönnin = the tooth (nominative)
• tönninni = the tooth (dative)

Since the sentence needs the dative after í, you get tönninni, not tönnin.

Aftur means again.

So the whole sentence means:

• My tooth hurts again
• I have tooth pain again

Putting aftur at the end is completely natural in Icelandic. Adverbs like this often come later in the sentence.

You could think of the structure as:

• Mér er illt í tönninni = My tooth hurts
• Mér er illt í tönninni aftur = My tooth hurts again

Yes, it is a very natural and common way.

Icelandic often talks about pain with this pattern:

• Mér er illt í ...

So:

• Mér er illt í tönninni is idiomatic and everyday Icelandic.

You might also hear other ways of expressing tooth pain, for example with words meaning toothache, but this sentence is absolutely standard and useful.

No, that is not correct.

The problem is that this expression requires the dative pronoun:

• mér, not ég

So you must say:

• Mér er illt í tönninni

Using ég would sound wrong because this is not a normal subject-based sentence like Ég er þreyttur (I am tired). It is an impersonal pattern: to me, it is painful in the tooth.

Not literally, no.

A more literal breakdown is:

• Mér = to me
• er = is
• illt = painful
• í tönninni = in the tooth
• aftur = again

So literally it is something like:

• To me, it is painful in the tooth again

But in natural English, we say:

• My tooth hurts again
• I have a toothache again

This is a good example of how Icelandic and English package bodily sensations differently.

A rough English-friendly guide is:

• Mér ≈ myehr
• er ≈ ehr
• illt ≈ ilt (with a crisp t sound at the end)
• í ≈ ee
• tönninni ≈ TUH-nin-nih or TUR-nin-nih depending on accent, with ö like the vowel in French peur or German schön
• aftur ≈ AF-tur

A few pronunciation notes:

• é is usually like ye or a clear long vowel similar to yeh
• ll in illt is not pronounced like English ll in spelling-based reading; just learn the whole word as sounding close to ilt
• ö is not an English o; it is a front rounded vowel, which can take practice

If you want a simple rhythm, try: MYEHR er ilt ee TUH-nin-nih AF-tur