Marjoram is sweeter and milder than its close relative, oregano. It's often used to season meats and fish, and works best when its added near the end of the cooking period. Fresh is best, but frozen or dried marjoram are acceptable substitutes. Don't confuse this with wild marjoram, which is better known as oregano. Opal basil has purple leaves and a longer shelf life than sweet basil, but the two can be used interchangeably in most recipes.
Mexican oregano has a mintier taste than ordinary oregano.
If you can't find it fresh, dried oregano is a good substitute. Use curly parsley if you want looks and Italian parsley if you want flavor. Parsley doesn't hold up well to cooking, so add it to cooked dishes at the very last minute. Frozen parsley is a good substitute for fresh, but dried parsley adds only color. The Italians are particularly fond of this pungent herb with its needle-like leaves.