Whole tomatoes give you more control
Whole peeled tomatoes can become a chunky sauce, a smoother sauce, or pieces in a stew. Crush them with clean hands in a bowl or break them up with a spoon in the pot. Keep the juices unless the recipe asks for drained tomatoes.
Look at the liquid when you open the can. Tomatoes packed in thick purée will behave differently from tomatoes surrounded by a looser juice. If your sauce starts unusually thick, add water a little at a time rather than assuming it needs more oil.
Diced and crushed tomatoes are different shortcuts
Diced tomatoes give you tidy pieces that often hold their shape through cooking. Add them to soup, beans, and dishes where distinct tomato chunks are welcome. If you want a completely smooth sauce, plan to blend them or choose another form.
Crushed tomatoes save the work of breaking down whole ones. The texture varies considerably by brand, from fine pieces to something close to purée. Check the can description and make a note after cooking if you want to buy the same texture again.
Paste needs a small job and a little heat
Tomato paste is concentrated, so measure it in tablespoons. Stir it through softened onion and oil for a minute before adding liquid. It should darken slightly and smell warmer, but it can scorch if left against a dry pan.
A tube is convenient when you use small amounts, though a can works well if you portion the remainder. Freeze extra paste in measured spoonfuls on a lined tray, then move the frozen portions into a freezer container. Label them so they do not become mysterious red cubes.
Taste the cooked dish before correcting it
Canned tomatoes vary in acidity, sweetness, salt, and consistency. Give a sauce time to simmer before deciding it needs sugar. A raw-tasting sauce may simply need longer, while a very thick one may need a little water.
Taste a spoonful with the food it will coat. Beans, pasta, butter, and olive oil all change how sharp the tomatoes seem. Add salt carefully, especially if the can includes it. Choose a can because its texture and ingredient list fit dinner, rather than relying on the prettiest label.
Keep in mind
For the smoothest quick sauce, choose crushed tomatoes or blend whole peeled tomatoes.
Freeze spare tomato paste in measured portions and label the amount.
