"My head hurts."

Translation: 頭が痛いんです。

[AlanS.6]

Why is there a 「ん」 after 「痛い」？

[Ever2662]

To expand on Rhiaaaaannon's explanation - の adds an "explanatory" tone to a statement, making it clear that it's either giving or seeking an explanation for something. For example, if someone asked you, "Why are you frowning?" you could answer 頭が痛いんです, making it clear that you're answering the question through your statement, rather than just making a totally separate statement about how your head feels.

I like to think of it in English as being analogous to adding "it's that..." before a sentence. My understanding is that this is what's literally happening in Japanese, too - the の is nominalizing the phrase, turning "my head hurts" into something a little like "(it's that) my head hurts". The second one sounds awkward in English, ofc, but I think it helps to capture the explanatory/nominalising tone that の/ん conveys.

Native speakers, people more clued up than I am on this subject - please feel free to correct me if I've grossly misunderstood what's going on here!

[Shinobusagi]

Good thing you expanded on that. If it were possible to give lingots in the app version, I would, but an upvote is the best I can do right now.

[Aelise_627]

Misa also explains this in her YouTube video if you'd like further explanation. Explanatory の/んだ...

[Rhiaaaaannon]

ん is like a contraction of の?. So this sentence more formally would be あたまがいたいのです。?の and ん, when used like this, don't really serve any grammatical purpose and are just kind of there to make the sentence... flow I guess?
But nobody really says あたまがいたいのです in speech so あたまがいたいんです is more natural.

[aninsultedcat]

In more formal Japanese, more so in writing, の is used in place of ん. When either seeking or giving an explanation, you can use んです to add an explanatory nuance to what is said.

なぜ頭が痛いんですか？「Why does your head hurt?」

ドアに頭をぶつけたんです。「I hit my head against a door.」

[Kigamoosh]

Do you have to add "ん" to make this an accurate translation?

[Derek003]

No. At least Duolingo accepted it without the ん. Going by the above, that would make it more like a simple statement (complaint?) than an explanation, I suppose..

[Liam161905]

Why is ん in: 頭がいたいんです,but not in: 目がいたいです

[Nekomanna]

You can say it withん, if you want to insist it is your eye or eyes that hurt. Withん、it's like saying "Matter of fact, my eye/head hurts".

[Toko102570]

「...なのだ 」and「...なんだ」means the same. The former is more formal than the latter. The latter is used more in the spoken language.
Reference: https:// oshiete.goo.ne.jp/qa/5199397.html