У and на - location versus motion

Both у (in/at) and на (on/at) have two different endings depending on the question they answer. For location (где? - where?), masculine and neuter nouns take a fixed -у ending, the same pattern days-months-a1 used for months: у Београду (in Belgrade), на столу (on the table). Feminine -а nouns take -и instead: кућа → у кући (at home).

For motion toward a destination (куда? - to where?), the noun instead takes the accusative pattern from cases-introduction-a1: masculine and neuter nouns stay unchanged, feminine -а nouns become -у. Идем у Београд. (I'm going to Belgrade) leaves Београд unchanged; Идем на плажу. (I'm going to the beach) changes плажа to плажу.