Pasta dishes can be so wonderful—incredibly light, unbelievably flavorful—but they can also be dense, stuck-together disappointments. You can help your pasta dish be its best—whether it's a baked lasagne, a pasta salad, or a slap-dash plate of spaghetti and pesto—by knowing a few of the hows and whys of cooking the pasta itself.
When you drop pasta into a pot of boiling water, the starch granules on the surface of the pasta instantly swell up to their maximum volume and then pop. The starch rushes out and, for a brief time, the pasta's surface is sticky with this exuded starch. Eventually, most of this surface starch dissolves in the water and washes away, and the pasta surface becomes a soft solid.
Stir at the start
Many pasta recipes begin like this: "Bring a large pot of water, 4 to 5 quarts, to a rapid boil." Do you really need this much water? Well, if you're only boiling a small amount of pasta (less than half a pound), you don't need so much, but a generous pot of rapidly boiling water is helpful for several reasons: it comes back to a boil faster when you add the pasta; it makes it easier to submerge long, rigid pastas like spaghetti; and it helps to reduce sticking slightly by quickly washing away the exuding starch from the pasta surface.
To keep pasta from sticking, stir during the first minute or two of cooking. This is the crucial time when the pasta surface is coated with sticky, glue-like starch. If you don't stir, pieces of pasta that are touching one another literally cook together.
Add salt, but not oil
You may have heard that you can avoid sticky pasta by adding oil to the pasta water. This can prevent sticking, but at a great price. Pasta that's cooked in oily water will become oily itself and, as a result, the sauce slides off, doesn't get absorbed, and you have flavorless pasta.
Adding oil may keep the pasta water from bubbling up and boiling over the rim, but this can also be achieved by making sure you use a large pot and also by reducing the heat a little (but still maintaining a boil). This is a much better solution than greasing your pasta and sacrificing flavor.
Salted water flavors the pasta. A generous amount of salt in the water seasons the pasta internally as it absorbs liquid and swells. The pasta dish may even require less salt overall. For a more complex, interesting flavor, I add 1 to 2 tablespoons sea salt to a large pot of rapidly boiling water. By the way, the claim that salted water cooks food faster (because of its higher boiling temperature) is exaggerated; you're not adding enough salt to raise the temperature more than about 1°F.