Control and summoning sickness

Question

If I cast Expropriate and my opponent chose money, if I were to target their Primordial Hydra, can I attack with it immediately?

Answer 1

No you can't attack with it immediately.

To be able to attack with a creature or pay {T} (tap) costs on its abilities, one of these two things must be true:

• You must have controlled the creature since the start of your turn, or
• It must have haste.

Since neither are true you can't attack with it straight away; you'll have to wait until your next turn.

Gaining control and haste is generally the domain of red theft cards such as Act of Treason: they take control of a permanent and give it haste so you can use it straight away. In exchange, those same effects usually aren't permanent, and the red player has to give it back at the end of their turn. Blue trades that "use immediately" upside for a "use forever" upside.