The word case has several meanings:

1. A container designed to hold and protect something. A pencil case keeps your pencils organized, a phone case protects your phone from drops and scratches, and a violin case cushions a delicate instrument. The case is built specifically for what it holds.
2. A situation or instance of something, especially one being investigated or requiring attention. A detective works on a case to solve a crime. A doctor examines a difficult case when a patient has unusual symptoms. When you say “in this case” or “in that case,” you're talking about a particular situation or set of circumstances: “In that case, we should bring umbrellas.”
3. The facts and arguments supporting one side of a debate or legal dispute. A lawyer presents her case in court, explaining evidence and reasoning to convince the jury. When you make a case for staying up later on weekends, you're offering reasons why it's a good idea. The phrase “case closed” means all the evidence has been presented and a conclusion has been reached.
4. In grammar, the form of a noun or pronoun showing its relationship to other words. “He” and “him” are different cases of the same pronoun: you write “he ran” but “I saw him.”