Because Russian has a free word order and nominative and accusative cases by masculine and neuter nouns are very often equal. Different nominative and accusative cases of animate nouns help us avoid ambiguity: The son (1) sees the father (2). - Сын (1) видит отца (2)./ Сын (1) отца (2) видит./ Отца (2) видит сын (1)./ Отца (2) сын (1) видит./ Видит отца (2) сын (1)./... . The son (1) sees the/a house (2). - Сын (1) видит дом (2)./ Дом (2) видит сын (1)./... . It is clear that the house cannot see the son, but if it could see, it would be сына: Дом (1) видит сына (2), but it is nonsense. You see, everything is logical, sensible and well thought-out in Russian.

Look at http://en.wikipedia.org/wiki/Grammatical_case Ancient Greeks imagined that a word stands directly (nominative case). When it begins to be used, it falls in less or more extent. Sometimes the declination extent is the same, so the word has the same endings. I'm doubting that it's easy to understand, but I think that's why this issue exists. Advise: Just memorize the endings

Когда русскоязычные учат падежи, то в родительном падеже подставляют слово «НЕТ» кого? чего? Нет чего? книги, дороги и т.д. Нет кого? девочки, человека. А в родительном падеже «ВИЖУ» кого? что? Вижу кого? девочку, человека или вижу что? книгу, дорогу. Так легче ставить слово в нужный падеж. Может это поможет почувствовать разницу и Вам.

It depends on the questions. Each case answers his own questions. The same questions give the same forms. Именительный (Nomin.) = кто? (a); что? (i) Родительный (Gen.) = кого? (a); чего? (i) Дательный (Dat.) = кому? (a); чему? (i) Винительный (Accus.) = кого? (a); что? (i) Творительный (Instr.) = кем? (a); чем? (i) Предложный (Prepos.)= о ком? (a); о чём? (i)

- For animate referents, the accusative forms are identical to the genitive forms (Acc=Gen). - For inanimate referents, the accusative forms are identical to the nominative forms (Acc=Nom). This principle is relevant for the following groups of words: 1. Masculine singular nouns ending with a consonant in the nominative form (the first declension) 2. Plural nouns (with no gender distinction). 3. Adjectives and words which behave as adjectives (pronouns - possessives, demonstrative etc., the numeral один "one", ordinal numerals, verb participles) related to a masculine noun in singular 4. Adjectives and words which behave as adjectives (pronouns - possessives, demonstrative etc., ordinal numerals, verb participles) related to a noun in plural, with no gender distinction 5. Cardinal numerals два "one", три "two", четыре "four"