Overdue and overdo look similar, but they mean very different things.
Overdue is an adjective. It describes something that should have happened already, or something that has not been returned or completed on time. You can use it for bills, books, assignments, and appointments. For example, My rent payment is overdue means the payment is late.
Overdo is a verb. It means to do too much of something. It often refers to effort, decoration, exercise, cooking, or emotional reactions. For example, Do not overdo the garlic means do not use too much.
A quick way to remember the difference is this: if you mean late, choose overdue. If you mean too much, choose overdo.
- The project is overdue.
- He tends to overdo his workouts.
One describes timing. The other describes excess. That small difference makes the right choice much easier.