how do you distinguish these?

Question

How do you distinguish these: grain, cereal, and lugume ?

Answer 1

Cereal and grain are of same kind. Cereal crops are grasses cultivated for their seeds (cereals). The word cereal derives from Ceres, the name of the Roman goddess of harvest and agriculture. Grains are traditionally called corn in the United Kingdom and Ireland, though that word became specified for maize in the United States, Canada, New Zealand, and Australia.
Eg: Wheat, red gram, black gram etc.

A legume fruit is a simple dry fruit which develops from a simple carpel and usually dehisces (opens along a seam) on two sides. A common name for this type of fruit is a "pod", although pod is also applied to a few other fruit types. Well-known legumes include alfalfa, clover, peas, beans, lupins and peanuts. A peanut is not a nut in the botanical sense; a peanut is an indehiscent legume, that is, one whose pod does not split open on its own.
Legume seed and foliage has a comparatively higher protein content than non-legume material, probably due to the additional nitrogen that legumes receive through nitrogen-fixation symbiosis. This high protein content makes them desirable crops in agriculture.

Answer 2

Grain is the gathered seed of food plants, Cereal is grain itself
I am sorry that i cant identify what is the lugume

Answer 3

just to finish the answer a legume is a bean.