This would be easier to remember if the English were "which country is between Spain and France"? "What" makes us think 'gì' or 'cái gì' while we've been taught 'which' is 'nào'.
what does "nằm"" mean?
To lie down. But it is also used as "to be located" in this case (this country is located at... - nước này nằm giữa...)
On the other, hand it is good to learn some variations that are actually used.
I have a hard time knowing whether or not to use "ở". Sometimes I see simply bên cạnh, sometimes ở bên cạnh. Would it mean fine to leave it out in this sentence? If not, why (is there a difference in meaning or grammatical context)?
come on! consistency please!
You can't say quốc già nào instead of nước nào?
yes you can.
ps. pay attention to diacritics, quốc gia not quốc già