is

is, ea, id is the workhorse third-person pronoun. Latin has no dedicated word for "he, she, it", so this is what fills the gap, and its great virtue is that it is colourless: it points back to somebody already mentioned rather than pointing anything out. That is what separates it from ille, which carries a "that one over there" flavour, and from hic, "this one here".

Learn it by case, because the case is chosen by the pronoun's job in the sentence: nominative is, ea, id; accusative eum, eam, id; genitive eius for all three genders, which is how Latin says "his", "her" and "its"; dative ei; ablative eo, ea. The plural runs ei (or ii), eae, ea, with eorum and earum in the genitive and eis (or iis) in the dative and ablative. The same forms double as a weak demonstrative adjective agreeing with a noun, as in is homo, "that man".