Päätäni särkee.

It is made of three parts:

• pää = head
• -tä = the partitive singular ending
• -ni = my

So päätäni means my head in the partitive case.

Because Finnish often uses the partitive with verbs that describe physical sensations or discomfort, especially when a body part is affected.

With särkee, the painful body part is very commonly put in the partitive:

• Päätäni särkee. = My head aches.
• Vatsaa särkee. = My stomach aches.

To an English speaker, this can feel strange, because English makes my head the subject. Finnish does not structure it that way here.

Because Finnish can show possession with a possessive suffix attached directly to the noun.

Here, -ni means my:

• päätäni = my head

So a separate word like my is not necessary.

You can add minun for emphasis:

• Minun päätäni särkee.

But in normal usage, -ni already does the job.

Because the sentence is not grammatically built around I.

In English, we often say something like I have a headache or my head aches. In Finnish, with this kind of pain expression, the verb usually appears in 3rd person singular:

• särkee = aches / hurts

So even though the meaning involves me or my head, the verb does not change to a first-person form.

From an English point of view, it may feel like it is, but in Finnish this is normal.

Finnish often uses this kind of impersonal/body-sensation construction without an explicit subject. English usually wants a clear subject like my head or it, but Finnish does not need to say one here.

So Päätäni särkee. is a complete, natural sentence.

Yes.

• Minun päätäni särkee.
  This is possible, but minun adds emphasis, because -ni already means my.

• Päätä särkee.
  This is also very common, especially when it is obvious whose head is meant from the context.

In everyday speech, Finnish often leaves possession less explicitly marked than English does.

Särkeä usually suggests an aching, throbbing, or ongoing dull pain rather than a quick sharp pain.

So it is a very natural verb for things like:

• headache
• toothache
• stomach ache
• sore muscles

A more general verb is sattua = to hurt.
So särkee is a bit more specific in feeling than just hurts.

They are very close, but not exactly identical in structure.

• Päätäni särkee. = My head aches.
• Minulla on päänsärky. = I have a headache.

The first describes the pain itself more directly.
The second names the condition as a thing you have.

In many real situations, though, they are practically interchangeable.

Päätäni särkee is the most neutral and natural order.

Finnish word order is fairly flexible, so Särkee päätäni is not impossible, but it sounds more marked and would usually need a special reason, such as emphasis or style.

So for a learner, Päätäni särkee is the best default pattern to use.