"I sprained my ankle yesterday" = This is correct. "I plastered my arm" = no. What injury are you trying to describe?

First phrase is fine, but you don't plaster your own arm. I suppose you mean after breaking an arm a doctor would put a "caste" on it. A caste is made with plaster.

For the second case, I think you mean that you broke your arm and an orthopedist put a plaster cast on it. In the United States we would say "My arm is in a cast." The cast is the hard shell that immobilizes the bone so it can heal. To describe the injury itself, we would say "I broke my arm" (informal) or "I fractured my arm" (more formal). There's another possibility. For a very small cut, you don't go to a doctor, you treat it yourself by putting an adhesive bandage on. I'm not sure, but I think that in the UK they call an adhesive bandage a "sticking plaster" and might say "I put a sticking plaster on my arm." In the US, one popular brand of ready-to-use adhesive bandages is "Band-Aid, and we would say "I put a Band-Aid on my arm."

Hey Sara, Yes, it's totally correct to say "I sprained my ankle yesterday". Other example: He stumbled and sprained a knee. You can also use the noun form: He hasn't broken anything - it's just a bad sprain. For the second one, you can say " My ankle was in plaster". Or, "My ankle was in a cast".