Marinate the Pork: Cut the pork tenderloin into cubes. Place in a bowl and marinate with 2-3 tsp soy sauce and a pinch of cornstarch. Set aside.
Mix the Sauce: In a separate bowl, whisk together the sugar, ketchup, soy sauce, salt, water (or pineapple juice), and rice vinegar. Set aside.
Prep the Vegetables: Peel and chop the carrot into 1-inch pieces. Remove seeds from the green pepper and cut into chunks.
Sear the Pork: Heat a skillet or wok over medium-high heat with a little oil. Add the pork cubes and fry until browned and cooked through. Remove pork from the pan and set aside.
Add carrots and bell pepper and stir fry for a minute or two. Remove veggies from the pan and set aside.
Simmer and Thicken: Pour the sauce mixture into the pan and bring to a boil. Add the carrots, green peppers, and pineapple chunks. Stir in the cornstarch/water mixture and simmer until the sauce thickens and becomes glossy.
Combine and Serve: Return the pork to the pan, tossing to coat thoroughly in the thickened sauce. Serve immediately over hot rice.