Herbs, both fresh and dried, are essential to cooking; if you have fresh herbs on hand, you’ll use them, and appreciate them. (Few dried herbs are worth keeping around, with oregano, marjoram, thyme, rosemary, and tarragon the notable exceptions.) Because of their distinctive flavors, no herb is a direct substitute for any other. There are many situations, however, in which you’re not necessarily looking for a distinct flavor but rather a certain freshness that an herb will provide. In these cases, of course, you can substitute parsley for basil, or cilantro for mint, and so on. Just don’t expect the end result to taste the same.
Fresh herbs keep best when stored in the refrigerator. For most, wrap them in damp paper towels and slip them into a plastic bag. For those with fragile leaves, like basil, chervil, dill, mint, or parsley, set them stem down in a jar of water with a plastic bag over the leaves, and change the water every couple of days. They’ll keep a few days or longer if they were very fresh to begin with.
Store dried herbs in tightly sealed jars, away from sunlight, for up to a year, but after six months or so, taste them before using to make sure they haven’t gotten musty.