What is the case? Nominative and accusative cases

You have already heard that the Russian language has cases which are awful and difficult. You might be surprised but there isn't anything like that! The cases are no more difficult than the prepositions in your language. Actually they are prepositions which are situated at the end of a word rather than before it.

Человек увидел кота - The man have seen the cat
Кот увидел человека - The cat have seen the man

In these examples the first word is in the nominative case, the third – in the accusative case. In the Spanish language, these examples sound like this:

El hombre ha visto al gato
El gato ha visto al hombre

See? There is also "a" – not at the end, but before the word!
For English-speaking students there is another example. In English there is also phenomenon that is similar to the accusative case, but only for pronouns:

He saw him

We cannot say "he saw he" because in that case we don't understand who saw and who was seen.
Russian words require the same modification.
Thanks to the cases we can rearrange words within the sentence as we want and the sense will be clear to us anyway!

Человек увидел кота = Человек кота увидел = Кота человек увидел = Увидел человек кота = Увидел кота человек

Now you just need to memorize what inflexion should be added to the words in the accusative case.

Человек: увидел человека
Кот: увидел кота
Дом: увидел дом
Папа: увидел папу
Здание: увидел здание
Дерево: увидел дерево
Трава: увидел траву
Девочка: увидел девочку
Дочь: увидел дочь
Тень: увидел тень
Часы: увидел часы
Ножницы: увидел ножницы

See, it is not always necessary to change the inflexion! Already easier!
Let's classify these words, look at them and draw a conclusion.

Человек, кот, -а
Папа - у
Трава, девочка – у
Дом -
Здание, дерево -
дочь, тень -
часы, ножницы,-

So, if the word is masculine, animate, and ends in a consonant – we add the inflexion "a". If the word ends in "a" – change it to "y" and in other cases there is nothing to change!

But there are some nuances, e.g. such words as:
Учитель – увидел учителя (because учитель+а = учителя)
Герой – героя (герой+а, й+а=я)
Отец – увидел отца (there is a mobile vowel; the number of such words isn't great, so you just need to memorize them)