Can any one tell me where is the place (like the patch of grass) for Penn students to lie down, study… Thank you so much!! 
In December? Good luck! Your best bet is to get some astroturf and put it in your room. There are a lot of patches of grass in May and September, from the Quad to the area around the Highrises (called . There’s a few spots around the library/College Hall area, down by the engineering building. Lots down below hollenback near the sports fields. And Fairmount Park has tons if it’s a nice day and you don’t mind a bit of travel. It’s not grassy, but the area around the entrance to the museum is quiet (usually) and has some benches and a nice fountain. Also Kaskey Park + Botanical Gardens (more pond than grass, but you could find a patch sometimes), just south of the Quad.
Mmmm your bedroom? I do remember having some sit and study sessions on the patch outside of College Hall.