A.  use The when we are thinking of a specific thing, and when it is clear in the situation which thing or person we mean
A1.  use The when there is only one of something: the longest, the world, the earth, the sun, the end of this month (e.g. The sun is a star)
A2.  the sky, the sea, the ground, the country, the environment, but space
A3.  the same
A3.  (go to) the cinema, the theatre, and the radio but television