How do you know when to use "ĉu" in a sentence?
What is the difference between "ĉu mi povas" and "mi povas"?
Using your example, "Mi povas." means "I can." This is a statement.
Adding "ĉu" to the beginning of a statement turns it into a question. So "Ĉu mi povas?" turns "I can." into "Can I?"
Mi trinkas. = I drink (or I am drinking). Ĉu mi trinkas? = Do I drink? (or am I drinking?).
"Ĉu" can also mean "whether", but I won't discuss that here because that wasn't your question.