I'm sure it can't be that simple..........this is Polish we're talking about!
Of course it can't be that simple. It's simpler! :D As a side note, I imagine that historically,
gdy probably appeared as an abbreviation of
kiedy, and in the closely-related Czech, the word for both is
kdy.
As a rule of thumb,
gdy tends to correspond quite closely to English
while or
as. On the other hand,
kiedy usually corresponds quite closely with English
when, but it's a forgiving word, and can also be used in both senses. So if
while and
when both make sense in English,
kiedy and
gdy will generally both make sense in Polish too.
So, saying, "While is the wedding?" in English wouldn't make much sense. Saying "Gdy jest ślub?" in Polish could also be met with a blank stare for the same reason, or it could be misinterpreted as a statement of surprise, "[You want to do that] while the wedding is going on!?"
On the other hand, "While I was a teenager, I used to babysit" still sort of makes sense in English, though English prefers to use
when in this context, whereas Polish isn't fussy. In fact, it might sometimes prefer to use
gdy or
while where English would prefer to use
when, but the two are quite often interchangeable. There may be a stylistic preference of one word over the other in certain situations, and as zetigrek said,
gdy can sometimes sound a little more posh, but usually, it's hardly a noticeable difference.
Generally speaking,
kiedy is the safer word to use, since it means both
while and
when. You don't usually have to use
gdy except in set phrases, such as
podczas gdy ("whilst", "at the same time as"),
w przypadku gdy ("in the event that").
Don't let the related word,
gdyby (the "if" in "if...would" sentences) confuse you. There is no corresponding "kiedyby" form. Only
gdyby exists.
And since I've brought it up, and to clear up any confusion before it arises, the difference between the two Polish
ifs is as follows:
gdyby is used in "if...would" sentences:
"
Gdybym była bogata, jadła
bym kawior na śniadanie."
"
If I were rich, I
would eat caviar for breakfast."
you could also switch things around and say:
"Jadła
bym kawior na śniadanie,
gdybym była bogata."
"I
would eat caviar for breakfast,
if I were rich."
Notice, that in English, "if...would" sentences are a special case too. Good English grammar requires the use of "If I were" instead of "If I was" or "If I would be," and the verb must be in the past tense. In fact, both Polish and English require the verb to be in the past tense, but Polish also uses a different word for "if" altogether.
In plain old "if" sentences that don't use the word "would", you use
jeżeli or
jeśli (their usage is identical.
Jeżeli is a little bit more posh):
"
Jeśli spadnie deszcz, zostanę w domu."
"
If it rains, I will stay at home."
tl;dr
gdy means
whilekiedy means
when or
while