Its body changes with the weather, and that is the way it reacts and moves.
1St move is Weatherball Depending on what the weathers like
By using Sunny day, Hail, Rain Dance. Moves that It can learn
Castform learns the move Weatherball, at level 30. This is the last move it can learn.
level 50
Castform does not learn Thunder as it levels up. Thunder is a TM that you can teach it. (In Pokemon Emerald, Ruby, or Saphire, you can get Thunder in the Lilycove Department Store.)
level 30
No, I don't think so.
You have to learn Weather Ball from the Move Tutor. You will need a heart scale though which you can get from underground.
it can learn Grass and Psychic moves
No, castform morphs due to the weather.
Argon is an element and so it does not learn any moves!
Of course it can learn dragon moves such as dragonbreath and dragon dance and also can learn moves like twister and waterfall.