"Are you a boy or a girl?"

[cricketswool]

'Kluk,' or 'chlapek,' 'mladík'? 'Holka,' 'děvče,' or 'dívka'? Are they interchangeable, or is there a way to know which one to use?

[endless_sleeper]

Kluk and chlapec are pretty interchangeable, kluk is more colloquial, I'd say. Hoch is quite formal. Mladík means a young man. Holka and dívka are pretty interchangeable, though dívka sounds more formal. Děvče is pretty old-fashioned. (Do note that děvče is a neuter, the same as German 'das Mädchen', the other words are masculine or feminine respectively.

[JanLyko]

'Děvče' and 'dívka' are totally interchangeable, just keep in mind that 'děvče' is neuter and 'dívka' is feminine. 'Holka' is more colloquial than both of them.

'Chlápek' is NOT a boy but "a guy / some guy".

'Chlapec' is more formal than 'kluk'.

'Mladík' means "young man / youngster".