populus

Populus, populi is second declension masculine and entirely regular. The singular is collective, the people as one political body, so it takes a singular verb however many humans are involved: populus Romanus vult, not volunt. The plural populi means separate peoples or nations.

Two traps. There is a second populus, populi, this one feminine, meaning "poplar tree" - same spelling, different gender, and the o of the first syllable differs in length. And in Christian Latin populus quietly narrows to the congregation, the laity as against the clergy, which changes the sense of a great many sentences in this corpus. Keep it apart from vulgus, the common crowd with a sneer attached, and from gens, a people defined by descent. Family: popularis, populosus, publicus (from an older poplicus), depopulor. English people, popular, populace, populate; Spanish pueblo, French peuple.