[weiping2007]

i heard liquid oxygen,liquid nitrogen....but i never heard liquid hydrogen before

do liquid hydrogen exist and what are the used?

[Yggdrasil]

Like most gaseous substances, if you cool the hydrogen enough and have enough atmospheric pressure, you can get hydrogen to liquefy.  Among other things, liquid hydrogen is used as rocket fuel (gaseous hydrogen would work also; however, liquid hydrogen is much more dense than gaseous hydrogen and therefore easier to store onboard the rocket).