Salazar had always been able to acclimate quickly; it was a quality that had saved his life many times. Being turned into a child and forced to live with an overbearing and intolerant family took slightly longer to adjust to than other challenges. He'd realized that he hadn't been transfigured when he started growing, which would have been impossible if he'd been transfigured.
He wasn't sure how he came to inhabit the body of this body but it was very inconvenient. While children seem to have boundless energy, it comes in bursts and they grow exhausted easily. His small frame was another annoyance; he was incapable of doing practically anything himself! Going from being one of the world's most powerful wizards to a young infant was truly a lesson in humility.
Despite his young age and incapable body, the family he was placed with often left him to fend for himself. The Dursley's were some of the most tiresome individuals he'd ever had the displeasure of encountering. Over-inflated ego coupled with rampant inferiority complex, Uncle Vernon and Aunt Petunia fed the other's problems while imparting the worst of themselves into their son Dudley. Obviously unhappy with being saddled with their nephew, they provided only the bare minimum for his survival.
While this normally wouldn't have been a problem for Salazar, he was skilled at hunting and would have been able to secure his own food and shelter, the world he found himself in was very different from the one he used to know. There were many inventions here he would never have even dreamed of: carriages that moved without horses, weapons with more destructive power and range than bows, creating light and heating without fire. The muggles had even managed to harness the power of lightning to power these things! Different laws and culture also presented challenges to him.
He'd first run into problems with these differences when he ran away from home at age four. Convinced he could care for himself better by himself, when the Dursley's took him on vacation with him that led them near a forest, Salazar took his leave of them to start his own life in the wild. When search and rescue found him a week later, they were surprised that he'd already managed to set up a reliable shelter, make a bow and arrows (among other weapons), as well as poach a deer. (He found out later it had taken them several weeks to find and disable all the traps he'd set up.) The only good thing that came from his excursion was his befriending of a snake, named Maas, he smuggled home. Uncle Vernon had not been happy with the fees Salazar's adventure had caused him to pay and that was the first time Salazar had been locked in his cupboard for an extended amount of time.
Large amounts of time with nothing to do had allowed him to focus on meditating and resulted in him unlocking his magical core at a much younger age than usual. Salazar had been overjoyed to find he still had magic and did his best to develop his young and currently rather wild magic. He was sure that word would spread and someone from the magical community would come to take him away like what happened in his time. This led to him demonstrating his magic as often as he could in an attempt to catch their attention.
After six months of continual attempts the only response he got was from the Dursleys. (He became very, very familiar with the inside of his cupboard.) He did his best to try and find out what had happened to the magical community but was unable to find anything. The general populace's opinion was that magic didn't exist; he knew that before he'd died Ryland had been trying to control the magic population and came to the conclusion that the war must have completely wiped out all the magical world. His sorrow at the discovery made him happy he had time to spend by himself locked in the cupboard as he was punished. It allowed him to grieve in relative peace. He did try his best to avoid public displays after that, but it turns out that children tend to have more volatile emotions than adults which resulted in several more magical demonstrations, unfortunate incidents that permanently colored a teacher's hair blue and turned Marge's terrible dog into an even more terrifying cat, before he managed to get his magic completely under control.
Ever the pragmatic, Salazar had turned his attention away from the magical world he'd known to the new world he found himself in. Regrettably, the laws that existed for his current reality meant it would be difficult for him to attain any sort of freedom until he came of age and was able to get out of the Dursley's 'loving' care. So he decided he needed to become far more successful in life than Uncle Vernon, if only to shove it in the Dursley's faces (he wasn't petty, not at all, he definitely wasn't researching Grunnings competitors in an attempt to ruin the company in the future.) It was about this time that he'd finally let go of his identity of Salazar Slytherin fully becoming Harry Potter; it helped that he started school and people actually called him by his name rather than boy or freak.
His teachers all praised him as a child genius which the Dursley's refused to acknowledge. They'd tried to convince the school Harry was cheating off Dudley, but he'd staved off that attack by framing Dudley for cheating on a test by planting the answer sheet in his bag. His cousin's following lackluster work put the final nail in the coffin for that story, and the teachers assumed any other attempts were simply overbearing parents with unrealistic expectations for their children.
His Aunt and Uncle instead tried to work him to death but he found it easy to simply "get lost" on his way home from school. Most of his time was spent at the local library; knowledge is power after all. He researched as much as he could and by the time he was eight had a working plan for the rest of his education and career.
Of course, as with all good plans there are always kinks. His plan to graduate and become an engineer was thrown for a loop when the summer he was to turn eleven he made an astonishing discovery. The summers were difficult for him to avoid the choirs his family forced on him. So he spent most of the vacation on yard work. His magical core had continued to grow and develop with his young body and as he was working he began to notice the magical presence of wards around the house. He searched the house and yard high and low before discovering the ward stone disguised as a decorative stone in the garden.
The implications of the ward stone were massive. It meant his previous assumption that the magical world had died out was incorrect; it also meant that someone knew that he was living here with muggles but instead of removing him to live with other wizards they had left him here placing wards on the house. He didn't know how to get in contact with the current magical community; most of the places he knew from the past had ceased to exist. Knowing that witches and wizards still remained but having no way to get in contact frustrated him to no end. The discovery provided him a distraction for most of the summer helping him through the multitude of chores assigned to him, through his cousin's birthday and trip to the zoo (where he most certainly did not use his renewed interest in magic not purposefully try and lock his cousin in an exhibit for shoving him.)
The culmination of his ruminating ended about a week before his birthday when for the first time in all his years living as Harry he received a letter. The day had begun normally enough with his Aunt and Uncle doing their best to ignore that he existed and Dudley doing his best to antagonize him, when the mail arrived his uncle had Dudley retrieve the mail (Uncle Vernon hadn't allowed Harry near the mail since several years ago when he had stolen several credit card offers and activated the cards in an attempt to get some spending money.)
"Harry's got a letter!" A cry came from the boy as Dudley waddled back into the kitchen.
Harry looked up to see his cousin waving a letter that appeared to be made from old fashioned parchment. Cursive writing covered the back and it seemed to be sealed by pressed wax rather than a modern adhesive. Dudley's movement made it impossible for him to make out what the writing said or what the crest pressed into the wax was. He snached the letter from his cousin before it could be handed to his uncle. He had time to read the back addressed to him, down to his bedroom (cupboard), and flip the letter to look at the crest before Uncle Vernon grabbed the letter from him. The impression in the wax made his insides freeze. That was the crest he had his friends had devised when they had started a school for magic; that was the crest for Hogwarts. He had thought it impossible for the school to have survived when magic had seemed to have all but disappeared. He looked up to demand his letter back in time to catch the looks his aunt and uncle were giving each other; the words froze in his throat as he realized they knew. They knew magic existed, and he was a wizard, and they never told him. All their actions over the years clicked into a new perspective as he realized they were trying to suppress his magic and heritage.
His uncle barred both him and Dudley from the room as the adults discussed what to do. Dudley, curious about their behavior (his father had never denio him anything), pressed his ear to the door trying to hear what was being said. Harry sat on the stairs watching; he felt frozen inside but his thoughts raced at a thousand miles an hour. Why were they only contacting him now? Had they been prevented before or just ignoring him? Why didn't anyone seem to know about magic seeing it obviously still existed? Was it a conspiracy? Were all magical children placed in homes that tried to stomp it out of them in an attempt to control magicals? His thoughts culminated into determination: he needed to get that letter. He didn't have enough information about the situation and the only way he could get more was the letter.
His head shot up as the door to the kitchen opened and his aunt and uncle came out. The consequential burning of the letter left him feeling empty instead of frozen. He was moved into Dudley's second bedroom despite his cousin's protests but couldn't bring himself to feel any sense of satisfaction at his distress. He didn't get much sleep that night and by morning had formed a new plan: he needed to get to Scotland. If the school still existed it probably was in the same place so if he visited he would be able to find the answers he needed. He would have to wait a couple days until Uncle Vernon withdrew the money for the vacation they had planned, then he would steal the money to make his way towards the northern part of the island.
He was planning his excuses to get out of chores the next day so he could go to the library and plan his route when the mail arrived again and with it three more letters from Hogwarts for him. These were also destroyed, and Uncle Vernon nailed the mail slot closed. Harry wasn't sure what this was supposed to do as the mailman would just leave the mail on the porch instead. But was amused the next day when twelve letters had been shoved through the cracks in the door. By now he recognized the spell being used. It was one the Godric had invented when they had been on military assignments in different areas. He had been too busy to answer his friend's letter so Godric had placed a spell on the letter so that everyday it was ignored more letters were delivered in increasingly annoying ways. His heart rose as he wondered whether his friends had also found themselves in this new world. It could explain the delay of getting in touch with him as they also acclimated to their new surroundings. He ignored the lingering voice in the back of his head that this was an illogical assumption because they had no way of connecting Salazar Slytherin with Harry Potter.
He relaxed as he realized his relatives had no way of getting rid of the annoying letters unless they let him read it and reply. Obviously he wasn't going to tell them that as he wouldn't be able to explain how he knew that, and it was terribly amusing to watch them respond to the rapidly escalating amount of letters. He chuckled to himself when they found letters rolled up in their eggs, and laughed openly when hundreds exploded from the fireplace, he was entertained when his uncle tried to throw off the letters by changing their location only to be thwarted when a thousand showed up at the hotel.
He was less amused laying on the cold damp floor of the shack and decided he should probably read one of the letters before his uncle decided to do something more drastic. It would be his birthday present to himself he decided as he drew a cake in the dust on the floor. He counted down the seconds until he turned eleven.
Midnight hit.
BOOM!