res

res is feminine and belongs to the fifth declension, the smallest of the five and one of only two common nouns in it (the other, dies, is also in this hundred). It runs res, rei, rei, rem, re in the singular and res, rerum, rebus, res, rebus in the plural. Genitive and dative are both rei, so context has to separate them, and the bare ablative re turns up constantly in de hac re, "about this matter".

The gloss "thing" badly undersells it. res is Latin's all-purpose noun for a matter, a circumstance, a fact, a legal case, an affair of state or property in general, and it anchors some of the language's most loaded phrases. res publica is literally "the public thing" and gives us "republic". res gestae are deeds accomplished, the title Augustus gave his own account of his reign. res novae, "new things", is the standard Roman euphemism for revolution. When a sentence seems to be about nothing in particular, res is usually the word doing the work.