Joku, jokin

At my work we often have very interesting discussions about the Finnish language.

The other day one of my colleagues used the word joku when referring to some object he needed to pick up from a shelf. I asked him that shouldn't he have used jokin instead? He was so convinced that he used it right, adding that actually jokin should be used when referring to people. I told him yes, it is right, but only in the spoken language, not in the so called "kirjakieli" - the literary language. He
strongly disagreed! We argued a bit, we almost bet on who is right, and finally searched it up on the internet.

As a side note, once the same colleague - a native Finn from the far north - claimed that the word optimistisemmat is written correctly as optimistisemmät - for him this is so much easier to pronounce.

In the literary language joku means someone and jokin means something (or some, one, a) So the former is used when referring to people, while the latter is used when referring to objects. Some examples:Anna se jollekulle! - Give it to someone!
Ota jokin niistä mukaan! - Take one of those with you!
Onko jokin hätänä?

When we go to the spoken Finnish things get more complex.
Jokin (its forms except the nominative, accusative - jonkin and partitive - jotakin) is used when referring to people:Anna se jollekin! - Give it to someone! (Lit.: jollekulle)
Kysyin joltakin, missä mä oon. - I asked someone where I am. (Lit.: joltakulta)

Joku is used for persons when in nominative (and sometimes in accusative when there is no confusion about the meaning).
Tuleeko joku mukaan? - Anyone coming with me?
Tunnetko jonkun Helsingistä? Tunnetko jonkin Helsingista? - both are used in spoken Finnish: Do you know anyone from Helsinki?

Joku (mainly in nominative and accusative - jonkun) is used in spoken language when referring to objects.Ota joku niistä mukaan! - Take one of those with you!
Otan jonkun mukaan. - I take one with me.
Onko joku hätänä?- Is anything wrong?

Generally, in spoken language the shorter between joku and jokin forms are used when there is no confusion about the meaning.

It appears to me that native Finnish speakers are so used to the spoken language that sometimes it is really hard for them to realize what the literary forms are. And sometimes they really don't know how to say it in the literary form.