Questions & Answers about Mér er illt í hálsinum, svo ég drekk heitt te.
Because Icelandic often uses an impersonal construction for physical sensations like pain, cold, hunger, and so on.
So Mér er illt literally works like To me, it is painful / bad.
- mér = to me in the dative
- er = is
- illt = painful / bad
This is different from English, where we usually say I have a sore throat or my throat hurts.
If you said Ég er illur, that would not mean I am in pain. It would mean something more like I am bad / evil / nasty / angry, depending on context.
In expressions like Mér er illt, Icelandic uses the adjective in the neuter singular because there is no normal grammatical subject.
This happens in a lot of impersonal expressions:
- Mér er kalt = I am cold
- Mér er illt = I am in pain / It hurts
- Mér er heitt = I am hot
So illt is not agreeing with mér. It is just the default neuter singular form used in this type of sentence.