Follow the rule of three when substituting dried herbs for fresh herbs. If substituting dry for fresh, use 1/3 of what the recipe calls for. If substituting fresh for dry, use 3 times what the recipe calls for.
For example, there are three teaspoons in a tablespoon – if a recipe calls for one tablespoon of fresh basil, you can substitute with one teaspoon of dried.
The rule of three works in most cases, but there are some exceptions, which is why it’s best to underestimate the amount you’ll need; you can always add more if the flavor isn’t right, but you can almost never subtract if you’ve added too much.
In addition, dried herbs lose their potency as they age. The older the dried herb, the more you’ll have to use. Smell the herb. No smell? It’s lost almost all of its potency; toss it and get a new one.
I’m an advocate of fresh herbs whenever possible – they impart the most flavor.