How do you say anything in Finnish?

Mai writes: 

​I want to know how to say and use "anything", "anywhere", "something", "somewhere" and the like. For example, "Ask me anything", "you can sit anywhere". Kiitos!

Moi Mai!

What an interesting question! I've been sitting on this for a while, as this is quite a big question that is a bit tricky to answer in a comprehensive way. Here's my best shot at it!

If we look at the English words in your question, we can see that there are two groups there: words that start with any: anything, anywhere  and words that start with some: something, somewhere. 

For the any-words, the Finnish equivalent is mikä vain (more formal, written language) and mikä vaan (more informal, spoken language). You have at least two options that also mean the same thing: mikä tahansa and mikä hyvänsä. As we're dealing with Finnish, we of course need to transform the word according to which form we need for a given sentence. In other words, mikä vain comes in a variety of cases to express different meanings. As for the choice between vain or vaan, hyvänsä and tahansa, they all mean the same thing coupled with mikä.  So here we go!

mikä vain/vaan
nominative (also known as the basic form or perusmuoto)
Mikä vain on mahdollista!
Anything is possible.

mitä vain/vaan
partitive (also known as the Finnish learner's nightmare)
Kysy mitä vaan!
Ask me anything!

minkä vain/vaan
genitive
- Minkä jätskin sä haluut?
- Ihan minkä vaan!
- Which ice cream do you want?
- Any one of them!

missä vain/vaan
​inessive
- Missä sä haluut istuu?
- Missä vaan!
- Where do you wanna sit? 
- Anywhere!

The same goes for all the rest of the local cases, so we get

mistä vain/vaan 
mihin vain/vaan
millä vain/ vaan
mille vain/vaan
miltä vain/vaan

and finally

minä vain/vaan
essive
miksi vain/vaan
translative

When we're referring to a person, we'll usually use kuka:

kuka vain/vaan
nominative 'whoever'
ketä vain/vaan
partitive 'whoever'
kenen vain/vaan
genitive 'whose ever'
kenet vain/vaan
accusative 'whom ever'
kenessä vain/vaan
inessive in who(m?) ever

and so on. 

Then we come to the second group, the words with some: something, somewhere and so on. 

something
jokin, in spoken language usually joku (which also means someone, to make matters more confusing)
Jokin muuttui suomalaisessa politiikassa.
Something changed in Finnish politics. 

Jokin also changes according to which case you want or need to use, giving you a whole bunch of new meanings.

For example:

Missä: jossakin
(in) somewhere
Mistä: jostakin
from somewhere
Mihin: johonkin
to somewhere

and so forth. To make sure that things don't get too simple, we also have some shorter versions to express the same meanings, for example:

Missä: jossain
(in) somewhere
Mistä: jostain
from somewhere

As you can see, there are quite a lot of options for expressing these kinds of meanings in Finnish, and I feel like I could just go on making lists about more and more subtle variation for the advanced learner who really wants to cover the whole topic. But I'll stop here this time, let me know in the comments if I've missed something that you'd like to hear more about!

EDIT 8.8.2018 at 19.07 Finnish time:  

My colleague Kati kindly pointed out a pretty crucial mistake in my text. I had written that the essive of mikä vaan is mikä vaan, but it's of course minä vaan. I've made the correction in the text also!

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