Quote Originally Posted by Pretty Butterfly
I still don't fully understand but I will try out some examples:

What type of car is it? = Какой (because I am asking about the qualities of something)
What time is it? = Что (because there are many, many possible times that it could be)
Which train are you taking? = Какой (because there are only a few possible trains that you could take)
What do you think? = Что (because there aren't a fixed number of opinions that you can have, it's a more "open" question).

Is that the difference? That что is used when the answer is very "open" and could be anything, wheras какой is used when there are only so many possible answers? Or am I still way off?
I think you are on the right track. The problem for English speakers is that although Что/Какой roughly corresponds to What/Which, the Russian and English versions do not overlap cleanly. You found a good example:

Which train are you taking?
What train are you taking?

Both are correct in English, but you can only use какой in Russian.

What train? = какой
What day? = какой
What color? = какой
What <noun>? = какой

What is usually что, but when you say "what" in the sense of "which one of <noun>", it's какой. Basically, if you can use both what/which in English, use какой in Russian. After some practice, you'll develop a feel for this.

Note: "What time is it?" is not a good example for this area of grammar because that question is rendered in a totally different way in Russian: Сколько времени?

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Now I have a question. How would I translate a "what" question with a definite article: (ex) "What is the plan?" I'm not asking which because there can only be one, but I want to know what the nature of it is. Что такое?