Note that "jako" is not a declension of the word "jak", it's a different word, and neither of them are the same as the word "jaki".

If I understand correctly, there are basically three ones that DON'T decline:

jako - "as" (On pracuje jako model - He works as a model)

jak - "how" (Jak to możliwe!? - How is this possible?)

jak - "like/as" (Jestem głodny jak wilk! - I am as hungry as a wolf!)

And then a fourth one, the only one which DOES decline:

jaki - what kind of (Jaki smak? - What flavor?)

Usually nominative.