Cumin can be added to a dish either as whole seeds or freshly ground, and the two forms behave differently in cooking. This distinction matters, because the same amount of seed will not taste the same depending on how it is prepared.
Whole seeds release their aroma in small, concentrated bursts. They are at their best when added to hot oil at the very start of cooking, so the heat releases the aromatic compounds — mainly the cuminaldehyde in the natural essential oil — into the oil and then into the rest of the dish. This is a common technique in Indian and Middle Eastern cooking, where seeds are often the first thing to hit the pan. In the finished dish, whole seeds also give occasional little pops of flavour and a slight texture contrast.
Freshly ground cumin behaves differently: it spreads evenly through the dish and creates a deeper, more uniform background note rather than distinct bursts. Grinding exposes more surface area, so the flavour is more immediate. You get the most vivid result if you grind just before use, because pre-ground cumin loses its aroma much faster. A small hand grinder or a mortar and pestle is enough — there is no need for an electric grinder.
Because cumin in both forms is very distinctive, start with a small amount. Too much can dominate a dish and hide the other ingredients. It is easier to add more at the end than to take it out once it is in.