The substantive in Indo-European has always three main categories which change its forms: the number, the case, the gender. It is known that the general trend of the Indo-European family is to decrease the number of numbers, cases and genders from the Proto-Indo-European stage to modern languages. Some groups are more conservative and therefore keep many forms, preserving archaic language traits; some are more progressive and lose forms or transform them very quickly. The Old English language, as well as practically all Germanic tongues, is not conservative at all: it generated quite a lot of analytic forms instead of older inflections, and lost many other of them.
Of eight Proto-Indo-European cases, Old English keeps just four which were inherited from the Common Germanic language. In fact, several of original Indo-European noun cases were weak enough to be lost practically in all branches of the family, coinciding with other, stronger cases. The ablative case often was assimilated by the genitive (in Greek, Slavic, Baltic, and Germanic), locative usually merged with dative (Italic, Celtic, Greek), and so did the instrumental case. That is how four cases appeared in Germanic and later in Old English - nominative, genitive, accusative and dative. These four were the most ancient and therefore stable in the system of the Indo-European morphology.
The problem of the Old English instrumental case is rather strange - this case arises quite all of a sudden among Germanic tongues and in some forms is used quite regularly (like in demonstrative pronouns). In Gothic the traces of instrumental and locative though can be found, but are considered as not more than relics. But the Old English must have "recalled" this archaic instrumental, which existed, however, not for too long and disappeared already in the 10th century, even before the Norman conquest and transformation of the English language into its Middle stage.
As for other cases, here is a little pattern of their usage in the Old English syntax.
1. Genitive - expresses the possessive menaing: whose? of what?
Also after the expression meaning full of , free of , worthy of , guilty of, etc.
2. Dative - expresses the object towards which the action is directed.
After the after the verbs like "say to smb", "send smb", "give to smb"; "known to smb", "necessary for smth / smb", "close to smb", "peculiar for smth".
Also in the expressions like from the enemy, against the wind, on the shore.
3. Accusative - expresses the object immediately affected by the action (what?), the direct object.
Three genders were strong enough, and only northern dialects could sometimes lose their distinction. But in fact the lose of genders in Middle English happened due to the drop of the case inflections, when words could no longer be distinguished by its endings. As for the numbers, the Old English noun completely lost the dual, which was preserved only in personal pronouns (see later).
All Old English nouns were divided into strong and weak ones, the same as verbs in Germanic. While the first had a branched declension, special endings for different numbers and cases, the weak declension was represented by nouns which were already starting to lose their declension system. The majority of noun stems in Old English should be referred to the strong type. Here are the tables for each stems with some comments - the best way of explaining the grammar.
This type of stems derived from masculine and neuter noun o-stems in Proto-Indo-European. First when I started studying Old English I was irritated all the time because I couldn't get why normal Indo-European o-stems are called a-stems in all books on Old English. I found it a silly and unforgivable mistake until I understood that in Germanic the Indo-European short o became a, and therefore the stem marker was also changed the same way. So the first word here, stán, is masculine, the rest are neuter. The only difference in declension is the plural nominative-accusative, where neuter words lost their endings or have -u, while masculine preserved -as.
A little peculiarity of those words who have the sound [æ] in the stem and say farewell to it in the plural:
Sing. Pl. Sing. Pl.
N dæg (day) dagas fæt (vessel) fatu
G dæges daga fætes fata
D dæge dagum fæte fatum
A dæg dagas fæt fatu
Examples of a-stems: earm (an arm), eorl, helm (a helmet), hring (a ring), múþ (a mouth); neuter ones - dor (a gate), hof (a courtyard), geoc (a yoke), word, déor (an animal), bearn (a child), géar (a year).