Early in his young life, Harry Potter was able to learn an important lesson that would propel him to success. In class one day Jeff Cole was asked where last night's homework was. Jeff pleaded, "I did it Mrs. Murray, I swear to God I did it, I must of left it on the kitchen table!"
To Harry's immense surprise Mrs. Murray accepted Jeff's excuse, even though it was clear to him that Jeff was lying. After class, Harry walked up to Jeff and asked him if he actually did his homework. Jeff responded, "Of course not, I just crossed my fingers so that the promise wouldn't count," Young Mr. Cole had no idea how greatly he had just changed the future.
A couple of days later, Harry was forced into testing out this new type of lie when he accidentally trampled Aunt Petunia's rose bed. Harry went to his Aunt and told her that her rose bed was trampled, but said (while crossing his fingers behind his back) , "I swear to God it wasn't me, it must have been one of Mrs. Figg's cats," Although he was still sent into his cupboard without food for a day for not preventing the travesty, Harry knew that one day was better than the week he had gotten in the past for the same offense.
Over time, Harry learned that it was best to interchange both truths and lies when he was swearing oaths, because he knew no one would believe him if he lied all of the time.
During the train ride to Hogwarts, one specific story from his new friend Ron Weasley quickly piqued his interest. This story detailed how Ron's twin brothers had attempted to prevent him snitching on them by performing something they called the "Unbreakable Vow". However, Ron said his dad had shown up when he had his hands clasped with Fred's hand, and he said his dad was the most angry he had ever seen him. This proved to Harry the importance that this new society placed on oaths.
After being caught flying when Madam Hooch told him not to by Professor McGonagall, Harry quickly said, "I swear to magic that I was just trying to stop Malfoy." After hearing an hour long lecture on the importance and dangers of magical oaths, Harry knew that he could game the system.
His first ploy was a simple prank on his most hated teacher, Snape in which he purposely misbrewed the boil-curing solution and slipped a couple of drops into the glass at the seat Snape usually sat at in the Great Hall. When Snape started to feel boils growing inside of his mouth he quickly blamed Harry because of the fact that it was a first year potion that someone had just pranked him with. Harry quickly stood up, crossed his fingers, and said "I swear on my magic that I didn't do it!" Harry figured that he wouldn't lose his magic, because one of the most important parts of any spell according to his teachers, was the intent behind the spell. Because of his crossed fingers, Harry clearly didn't intend for Magic to uphold his oath, and therefore he wasn't liable for the penalties.
For expedience sake, we will simply give you Harry's most important truthful and not so truthful oaths from each of his next seven years. You can fill in the rest with your own imagination.
Year 2:
"I swear on my magic that I am not the Heir of Slytherin"
"I swear on my magic that I won't hurt you Mr. Basilisk"
Year 3:
"I swear on my magic that Sirius Black is innocent"
"I swear on my magic that I did not know where Professor Snape went last night"
Year 4:
"I swear on my magic that I didn't cheat in this Tournament"
"I swear on my magic that I didn't enter myself in the Tri-Wizard tournament"
Year 5:
"I swear on my magic that Voldemort is alive"
"I swear on my magic that Voldemort returned from death's clutches"
Year 6:
"I swear on my magic not to abuse any magic that you teach me"
"I swear on my magic that Malfoy is up to no good"
Year 7:
"I swear on my magic that I'm forced to have multiple marriages"
"I swear on my magic that I love you"
Surprisingly, the second of each of these oaths is actually the truthful one.