DECLENSIONS AND CASES:

Modern English is an analytic language. It primarily makes meaning by word order. To show that a word is in the nominative case, (i.e., the word functions as the subject of a clause), modern English speakers put that word in front of a verb. To show that a word is in the accusative case (functioning as a direct objective), modern English speakers put that word after the verb. For instance,
The teacher (nominative) graded the tests (accusative/direct object).
Word order thus becomes very important in analytic languages.
However, Anglo-Saxon (Old English), Latin, Greek, and many other languages are or were synthetic. These languages require that the case or function of each word be visibly marked through inflections or declensions.
In synthetic languages, word order does not matter. Synthetic languages primarily show case by inflecting words (i.e., changing the form of the words in pre-established patterns called inflections. Often this takes the form of some special ending added to the word or its stem. Such special endings are often called declensions by teachers of Latin, Greek, or Old English. Most synthetic Indo-European languages make use of the following cases:
Nominative Case: Words in this case usually function as the subject of a sentence, or in some cases as a predicate nominative. For instance, "John arrives tonight" would require the word John to be in the nominative case since John functions as the subject of the clause. On the other hand, "It is I, Hamlet the Dane," would require both the word I and Hamlet to be in the nominative case also, since these are functioning as predicate nominatives for the subject it.

Accusative Case: Words in this case commonly function as the direct object of a verb, though often certain prepositions will require an object of the preposition to be in the accusative case. For instance, "Darth Maul struck Obi-Wan" would require the word Obi-Wan to be in the accusative, since that poor Jedi is the object directly affected by the verb struck.

Genitive Case: Words in this case are functioning in a possessive manner, though often certain prepositions or special verbs will require an object to be in the genitive. In English, we often show this relationship by either an apostrophe 's or we create it artificially by using the pronoun of. For instance, we might see either "This is Bob Miller's house," or we might see "This is the house of Bob Miller." Synthetic languages would convey the same idea by putting the name "Bob Miller" in the genitive case. More rarely, some Indo-European languages might use the genitive of material to indicate the material substance of an object. Thus, English speakers refer to Superman as "the man of steel" or architects speak of "a house of stone."

Dative Case: Words in this case are functioning as the indirect object or the recipient of a direct object, though often certain prepositions or special verbs will require an object to be in the dative. For example, in this sentence, "Carla gave Sandy a gift," the word Sandy would be the indirect object or recipient, and thus that word would be in the dative case.

Ablative Case: Words in this case typically indicate source, origin, separation, or causation, though certain prepositions or special verbs will require an object to be in the ablative. For instance, "He came from Mantua" would require the word Mantua to be in the ablative of origin. Likewise, "He left Mantua at 2:00 pm" would require the word Mantua to be in the ablative of separation. "Because of rain, he left," would require a synthetic speaker to use an ablative of causation for the word rain.
Vocative Case: Words in the case typically indicate that the word is being specifically addressed or spoken to. For instance, consider this sentence: "John, would you be a dear and take out the garbage?" In this example, the word John would be in the vocative case in a synthetic language.
More rarely, some Indo-European languages like Sanksrit, Old Norse or Anglo-Saxon may use these cases as well:
Locative Case: Words in this case function to show location; for instance, "Joe went home." The word home would be in the locative case in a synthetic language. Many synthetic languages simply use the dative case here.
Instrumental Case: Words in this case are functioning to illustrate how or by what means an action was taken; for instance, "Joe smashed in the door with a hammer." The word hammer would be in the instrumental case in many languages to show what means Joe used to smash in the door. [Note that many Indo-European languages simply use the ablative case here.]
Interjective Case: Words in the interjective case are outbursts or exclamations separate from the rest of the sentence's syntax. Examples in English might be, "Gee-whiz!" or "Yikes!" or "Golly" or "Damn!" or "Ah!" Some languages would put these interjections in their own separate case, but most simply use nondeclinable words for interjections