Gases can be changed into liquid state by the process of condensation.
Any gas can be liquified by means of temperature and pressure.
The temperature and pressure at which the gas gets liquified is said as critical temperature and critical pressure respectively.
These are related to the boiling point of the gas to be liquified.
It is simple, as liquid has molecules more closer than gas, if you want to liquify a gas, you have to make the molecules come closer to each other sufficient enough that the weakest of the forces( Wander walls forces) come into effect.
And just pressure dosent make a gas liquid, as you will have to reduce the rate of drift, enthalphy, and etc. of the atoms and molecules so that, they can remain closer.
It is same as if you are calm and silent, then only you will sit quitely besides someone in a class or everyone would just winder here and there!
Hope this answers your question. Comment if you want more explanation.