These three turn up under more dinners than anything else, and almost everyone cooks at least one of them slightly wrong for years. None of them is difficult. Each has two or three rules that do all the work, and the rest is preference.
Rice
Rinse it until the water runs nearly clear unless you want it sticky. That rinsing removes loose starch, and loose starch is what makes rice gluey. Use a ratio rather than a saucepan-full of water: roughly one and a half parts water to one part long-grain rice. Bring it to the boil, put the lid on, drop the heat as low as it goes, and do not lift the lid for twelve minutes. Then take it off the heat and leave it alone for another ten with the lid still on. The resting is not optional — it is where the grains finish and separate.
Pasta
Plenty of water, properly salted — it should taste seasoned, not like the sea. That is the only chance the pasta itself has to be seasoned from the inside. Cook it a minute less than the packet says and finish it in the sauce, in the pan, with a ladle of the cooking water. That starchy water is what makes a sauce cling instead of sliding off. Never rinse it, never add oil to the water. Both stop sauce sticking, which is the opposite of what you want.
Potatoes
Start them in cold salted water, not boiling. Dropped into boiling water, the outside cooks to mush before the middle is done. Cut them to an even size and salt the water properly, the same as pasta. A potato boiled in unsalted water cannot be rescued with salt afterwards. For roasting: parboil, drain, then shake them in the dry pan until the edges scuff. Those scuffed edges are the crisp bits. Into hot fat, in a single layer, and leave them alone.
The rules they share
Salt the water, not just the finished dish. Anything cooked in water is seasoned from the water or not at all. Keep the cooking water for pasta and potatoes. Pasta water thickens a sauce; potato water enriches bread and soup. And cook more than you need. Cold rice makes the best fried rice, cold potatoes fry better than fresh ones, and cold pasta is a salad. Each of these is genuinely better on the second day if it is cooled quickly and stored properly.
Rinse rice and let it rest. Salt pasta water and finish in the pan. Start potatoes cold and rough up the edges. That is nearly everything, and it is the difference between a side dish and the best part of the plate.