As a Latin student begins to make progress through the five (5) Latin noun declensions, he or she will soon begin to notice—maybe with some degree of consternation—that there are some noun endings that repeat themselves in different “cases” even within the same declension (e.g, the –ae ending used in three different locations [cases] within the 1st declension). As well, there may be repetition of the same case ending from one declension to the next, even involving different genders and different cases (e.g., the –ī ending that occurs as twice within the 2nd declension masculine [genitive sing. and nom. pl.] and once in the neuter [genitive sing.], and then again in the 3rd declension as a dative singular). Even the 2nd declension -us masculine nominative singular ending is repeated elsewhere—four times—within the 4th declension noun structures!
But, do not fear and tremble unnecessarily, even over something as seemingly ominous as this! “Let the vocabulary bell ring inside your head: when you think about how you first wrote, saw, and said the original form of the word, it will help significantly in directing your brain (the most sophisticated “computer” on the planet) first to the appropriate declension, and then to the appropriate case usage for the context which you are observing.
So, remember: “Let the vocabulary bell RING!”