Answer :
1. Nature
2. Time
The subject of a sentence is the person, place, thing, or idea that is DOING or BEING something. In this case, nature is BEING the best physician and time is BEING a great healer.
2. Time
The subject of a sentence is the person, place, thing, or idea that is DOING or BEING something. In this case, nature is BEING the best physician and time is BEING a great healer.