DECLENSIONS
Latin nouns are grouped into classes called declensions. These bring together those words which have the same endings for the various cases, numbers and genders. If you know that rosa (rose) and ursa (bear) are both placed in the first declension, then you know how they will change in different contexts. The genitive plural in the first declension changes –a to –arum. So odor rosarum means “the smell of roses”. You know that ursa is also a first declension noun, so you know what odor ursarum means.
There are five declensions. Three cover the vast majority of nouns. The others are almost an afterthought, but they do contain some basic words such as manus (hand) and dies (day). You can choose to learn all the endings by rote, or to refer to a crib-sheet until they come naturally to you. You will find just such a sheet in the resources section.