How do I know which ending to use after the preposition «в»?

The preposition «в» has two meanings: in/on or to (as in movement, going to a place). When it means in or on, the noun which follows must be in the prepositional case, ending in «е». For example «мальчик в парке», “the boy is in the park”. When «в» means to, the noun which follows must be in the accusative case. This means that for masculine nouns their form is the same as the nominative, but for the feminine form «а» changes to «у» and «я» changes to «ю». For example «Я хожу в театр» (I go to the theatre) but «Я хожу в дискотеку» (I go to the disco).As a general rule, “When «в» means to change «а» to «у» and «я» to «ю», that’s all you really have to do!”