What is the subordinate clause in this sentence?

As I was walking, I stumbled upon a beautiful seashell.

The subordinate clause is As I was walking .

the metamorphosis is based on an evident father-son problem. this story shows, as a result of the unthinkable transformation of the son in an insect, the deformation and destruction of the individual caused by the authoritarian and hierarchical structures of the family. the narrative begins with the transformation into an insect of enormous proportions of the traveling salesman gregor samsa, a lonely, introverted and submissive, who keeps his family with his work. when he worked, he was appreciated by everyone, but, now that he can not work, he is despised, especially by his father, but this is going to be joined, painfully for the protagonist, first his mother and, finally, his own sister, grete, with whom he had held a special relationship of complicity and affection for many years.

condemned by a deep sense of responsibility, gregor fears the reaction that his frightful aspect will incite in his family. he feels guilty for not being able to continue being useful to the beings he maintained. his own family, however, is ashamed and runs away from him, and even tyrannizes and resists him. the father, of a severe and despotic character, humiliates him and treats him with great cruelty. all his attempts to escape from the confinement to which he is subjected in his own room fail before the hostile rejection of the beings that surround him. prisoner in his lair and unable to continue playing the only function of making money he had, finally chooses, with the consent of his parents and sister, to die of starvation. with this, he is freed from a servile and unhappy existence. the family appreciates gregor while he fulfills the interests. but when he is unable to continue earning money, it becomes a problem, a shame, and family relationships begin to become tense, until coexistence becomes impossible.