A boiled egg may seem like an easy thing to prepare, but this simple cooking process has a surprisingly high margin for error. The potential for mistake lies in the difficulty inherent in getting the cooking times just right, not only in terms of number of minutes but in how you prepare the water.
Preparation
In order to get the boiling time of the egg just right, it's important to prepare the boiling water correctly. First of all, be sure to use enough water in the pot not only to completely cover the egg, but to give an inch or two of coverage above it. This way, the egg will stay submerged and get thoroughly cooked even as some of the water cooks off.
Also, be sure not to add the egg to the heating water or start timing it until the water is fully boiling. Boiling means that the water is bubbling vigorously all across the surface. You'll also want to add a few pinches of salt to the water; this won't affect the timing, but it will keep the egg from spreading out if it cracks a little.
Soft-Boiled
Making a soft-boiled egg is a good option if you want an egg with a yolk that's runny, semirunny or just on the soft side, but you don't want to deal with the added oil that comes with a fried egg or the difficulty associated with poaching an egg. Add the egg/s to the boiling water and set a timer depending on how you want your egg to turn out.
For a very soft egg with a completely runny yolk, cook the egg for four minutes. If you want your yolk half runny, half firm, cook it for five minutes. For a yolk that's just a little soft, cook your egg for six minutes. Be sure to remove the egg from the water right away after your timer runs out. (A slotted spoon is good for this.)
The number of eggs you cook at one time won't affect the cooking time, but if you have unusually large or small eggs, they may take slightly more or less cooking time, respectively. The difference should be fairly minimal, however, and you'll only need to adjust by seconds either way to get the results you want. Err on the side of less cooking time, since you can, if necessary, return your peeled egg to boiling water (provided it's salted) for some additional cooking.
Hard-Boiled
A hard-boiled egg is an egg with a fully hardened yolk. It is cooked in the same manner as a soft-boiled egg, only for a longer period of time, usually in the range of 10 minutes or a little more. Hard-boiled eggs require less precise timing than soft-boiled eggs, but if you cook them too long, the yolk will get a layer of green tinge around its outside. However, this does not affect the taste of the egg. If the color doesn't bother you, go ahead and err on the side of too long to ensure that your yolk has no trace of softness left in it.
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