This was obviously a long time ago, given the English. The character seems to be upset and he finds this loss to be a great one. He clearly loves this girl and he thinks losing her will be a tragedy. He is calling the girl the 'sweetest flower' and the 'untimely frost' is her death that happened too soon, with her being so young.
The complete prepositional phrase in the sentence stated above is "in the door's key hole". As the name suggests, this type of phrase is introduced by prepositions which in this case is with the use of preposition "in". It answers the question "where" which refers to the location of the object it modifies.
he brought home an apple because he thought the color was beautiful. His parents disciplined him because at that age he shouldn't have been able to see color
