Since cilantro is the same thing as coriander why not use it as the substitute?

Answer 1

Coriander is a plant. Cilantro is a term referring to coriander in the broadleaf growth stage prior to bolting. Use of the name cilantro is uncommon outside of North America.

In the broadleaf stage of growth, the entire plant is used, root, leaf and stem. It is mild in flavour and not at all bitter. This is what is required when a recipes specifies cilantro in the United States, or coriander in the most of the world.

When the ground seed is required this should be specified as "ground coriander seed" but sometimes in the US, the seeds (actually dried berries) are referred to as coriander. No doubt this is a source of culinary confusion.

The stages of growth produce similar flavours. The seeds are more pungent than the leaves, with a stronger citrus flavor. They may be slightly bitter depending on soil acidity.

You may have heard the leaves described as bitter. What this shows is that someone tried to used coriander that had bolted (exited the cilantro stage as the plant prepares to flower). Bolting coriander has finer, sparser, darker foliage with unmistakable light/dark striation along stems. The plant is spindly, and takes on a bitter flavour with a soapy texture.

If planning to substitute, be aware of these differences. Especially keep in mind that because the flavor of the seeds tends to be stronger than that of the leaves, you'll need substantially less of them. Don't be afraid to experiment, and have fun.