1. Not choosing the right shape
Pasta doesn’t come in 101 shapes and sizes just for fun. There’s a purpose for nearly all of them (okay, maybe not this one), and they aren’t always interchangeable. If you don’t have the pasta shape called for, make sure to swap in a similar shape. Think of them in terms of categories: big tubes (penne, rigatoni), little scoop-y/tube-y shapes (elbows, shells), twirly spirals (rotini, gemelli), thinner strands (spaghetti, linguine), wider strands (fettuccine, pappardelle). Then, try to keep things in the same family.
2. Blindly following the timing on the box
Think of the time listed as a suggestion, not the Gospel. After cooking more than a thousand pots of pasta, I’d say the box is accurate only about 50% of the time. It’s rare that your pasta will be overcooked when the timer goes off, but it can be severely undercooked — and it’s not necessarily the company’s fault. A lot of factors that go into the rate at which your pasta cooks: the heat retention of your pot, how quickly your water returns to a boil after your pasta hits the water, the strength of your burner, etc. Just to be safe, when the timer goes off, fish out one or two pieces and taste them. Depending on how undercooked it is, you’ll want to continue in 30-second to one-minute intervals, tasting along the way. Remember, you can always continue cooking, but you can’t undo a mushy noodle.
3. Overcooking pasta when baking
Speaking of mushy noodles … on the flip side, always undercook your pasta if it will also spend more than 10 minutes in the oven, since the pasta continues to cook as it bakes. That way, by the time the casserole is done, the pasta is perfectly al dente.
4. Not salting the cooking water enough!
This is, hands down, the biggest mistake home cooks make when cooking pasta. The boiling stage is the only point that you get to season the pasta throughout, and to get in the noodles, the water has to be very, very salty, like mouthful-of-ocean-water salty. My personal rule of thumb is to throw in a small palmful of salt for 1 lb. of pasta in a 7- or 8-quart pot. No need to worry about using that much salt, most of it will be going down the drain anyway. And that leads me to…
5. Dumping all of the cooking water
When it’s time to drain your pasta, don’t pour all the water out. That’s valuable stuff! You probably won’t need it for super saucy preparations (think marinara or bolognese), but for anything that’s a bit drier (like olive oil-based sauces) or creamier, adding a splash or two of cooking water is the perfect way to take clumpy and dry sauce to luxurious and silky. The water helps loosen up the sauce so it can coat every noodle, while the starch in the water helps it cling to the pasta better.