Just played after a long time, and I saw that some cards go like "If you have X, then Y" or "if you control X, then Y" where X is a card class like "Mech" or "Beast". What do these terms mean exactly?
The "if you have" wording in terms of "if you have a beast" means that the battlecry / secondary spell effect only triggers if one of your minions on the board is a beast.
The "if you control" terminology is only seen in terms of beast/mech in the newest adventure, the League of Explorers. It works identically to the "if you have" wording. This is one of many examples of inconsistent wording between Hearthstone cards that work identically.