simply put, czy means "if" or some variation of "if". Like is or are.
Just remember that when a question begins with czy, it was allways recieve a yes or no (tak, nie) answer.
OK, I'll try to clean this mess. Meanings of "Czy":
1. Yes/No question marker.
If you take a declarative sentence ('Kasia jest zmęczona' = 'Kate is tired'), it is enough to add 'czy' in the beginning to turn this sentence into a question ('Czy Kasia jest zmęczona?' = 'Is Kate tired?'). In case of question marker function, 'czy' can be omitted ('Kasia jest zmęczona?='Is Kate tired'). The intonation is enough to make it a question. As a question marker, 'czy' has no English counterpart.
2. 'or' in questions
Napijesz się kawy, czy herbaty? = Will you drink tea or coffee?
3. 'if/whether' introducing a subordinate clause
I don't know if I have seen this movie = Nie wiem, czy widziałem ten film
Other meanings are not especially important.
I would say don't confuse "czy" with "is/are/will/etc." that is used in English sometimes to start a sentence? That is either already acounted for in the Polish by "jest/bedzie/etc." or it is "hidden" in the Polish verb (because Polish actually has verb forms for perfective vs. imperfective unlike in English where we have to use helping verbs).
I would disagree with the Spanish analogy as well, because one can also make such constructions as "¿Que ...?" which would then be "Czy co ...?", and I don't know if that would make sense.
I think of "czy" as Polish for the "missing word" that belongs to the set: what, who, where, when, etc.. I would venture to say that, when "czy" begins a question, it simply has no English translation. Haha, this is payback for a/an/the, except that we English speakers only need to be aware of it and don't have to worry about when to use one of three different versions.
That reminds me, that is another thing I never got the hang of. When to use "czy", "albo", and there was some third word. I would translate all of them to "or".
The third word is "lub".
