영어 문장에서 주어 찾는 것 좀 도와주세요

아래 문장에서 주어를 찾아 밑줄 그어야 하는데 잘 모르겠어요.
도움 부탁드려요.
1. In the fifteenth century an Italian scientist invented a way of writing a message on the white of boiled egg.
2. Overnight fans slept outside the stadium to make certain they could get tickets.
3. Everything was silent and then there came a strange knocking at the door.
4. There are many important issues facing the world today including unemployment, health care and overpopulation.