LINGVA LATINA
 
  Latin is an inflected language. Unlike English, words change according to their functions in Latin. Though, some words change in English as well.
For instance:
       I see you, but you can't see me.
  In this sentence, "I" is replaced by "me". (objective pronoun)

  In Latin, nouns, pronouns, and adjectives change according to their functions in a sentence. In this process, endings of nouns change with a number of regular patterns. These patterns are called declensions. There are 5 declensions of Latin nouns which I will mention later.  Now, we will take a look at cases in order to learn how they function in a sentence.  There are 7 cases which are nominative, vocative, accusative, genitive, dative, ablative and locative. (note: 'locative case' is rarely used.) 6 cases are used frequently.
 Another important point to mention is that case of a word also varies according to its gender (masculine, feminine, and neuter), singularity, and plurality of it.
I will demonstrate only how cases operate in a sentence. So, don't worry if you don't figure out everything.

1. Nominative Case

  It is the case of subject in a sentence. (note: highlighted word is in nominative case)
    Marcus laborat.
    Mark works.

2. Vocative Case
 It is the case which is used to address or call someone or something.
    o fortuna!
    Oh fortune!

3. Accusative Case
 
It is the case of object in a sentence.
     Quintus Marcum salūtat.
     Quintus salutes Mark.

4. Genitive Case

 It is used, as 'of' in a sentence, or  to show possession.
    bellī deī.
  
War of gods.

5. Dative Case
 It is used, as 'to' and 'for'.
    lactem felibus do.
    I give milk to the cats.
 
6. Ablative Case 
 
It is used, as 'by', 'with', 'from', 'at', 'in', or 'on' in a sentence. Ablative case operates like a proposition. However, it can be used with a preposition as well. In classical Latin, usage of propositions with ablative case is less frequent than in Vulgar or, Medieval Latin. Moreover this gives us more clues about evolution of Latin through oromance languages. I will give examples in advanced posts.
    Marcus vīnum forō emit.
    Mark buys vine from the forum.

7. Locative Case
 It is used to indicate the place where something is happening:
    Rōmae
    At Rome