Ksenia is right, it depends on which case you're going to use. However, there is one thing you should understand: usually there are different endings for feminine and masculine nouns, but that's not the only criterion. The way noun endings change depends not only on their number and gender, but also on the declension (grammatical group) each noun belongs to. In Russian there are three of them. You can find more information here: http://masterrussian.com/aa052000a.shtml Speaking about your question, the latter is an example of a first declension noun change (Nominative case -> Accusative case). Москва -> Москву, земля -> землю, юноша -> юношу. The first one, however... Well, I'm not sure if there is any feminine noun whose ending changes from (a) to (o). Could you provide an example, please?

In the Russian language has 3 kind. Feminine gender : -а -ая Neuter: -ое Masculine: ..... В русском языке 3 рода. Женский род : -а -ая Средний род: -ое Мужской род: .....

когда можно сказать : Москву, Москве, Москва? "Москву" можно сказать когда вы идете в Москву. Или прилетаете в москву. В этой форме " Москву", в Английском языке используется предлог " То". "Москве" это форма используется когда вы находитесь в Москве. Например: Я сейчас в Москве. " Москва" это уже базовая форма, без изменение. Например: Москва это столица России. when we can say : Moscow, Moscow, Moscow? "Москву" you can say when you go to Moscow. Or fly to Moscow. In this form,"Москву" , in the English language is the preposition "To". "Москве" is the form used when you are in Moscow. For example: I now in Moscow. " Москва" is already a basic form, without change. For example: Moscow is the capital of Russia. Sorry for my mistakes. I do not know much English.