It has been puzzling me for quite a while and no online explanations seem to give me a thorough understanding can someone help please?
Nominative (Nom) is generally considered the default case and hence is the form found in dictionary entries and it’s used for the subject of a clause. In almost all cases, at least one attribute, i.e. a determiner (DET: pronoun PRON, article ART) or “strong” adjective (ADJ), will carry the characteristic ending for the gender/number of the substantive (SBST): +r masculine singular (Masc), +s neuter singular (Neut), +e feminine singular (Fem) and +n or +e plural (Pl).
Accusative (Acc) only applies to the masculine gender in singular (Sg) where it always requires an +n ending for attributes, to a single inflection class of nouns, and to 1st and 2nd personal pronouns. Otherwise, Acc is equal to Nom. It’s most often used for direct objects, but verbs and prepositions can demand other or multiple cases, too.
Nom and Acc are considered the direct cases (Dir). They contrast with the oblique cases (Obl), which are usually marked stronger by inflection suffixes, except for weaker adjectives which are always +n.
Dative (Dat) is always the same for masculine and neuter with a characteristic +m suffix in the strong attribute (DET or ADJ). Together, they’re sometimes considered the standard genders (Std). Reflexive personal pronouns fuse Acc and Dat (m/d/sich) and singular nouns have usually the same form in both cases, too, but some DatMasc nouns support an (now optional) +e suffix. DatPl has an +n wherever possible. The case is often used with indirect objects, many prepositions require it.
Genitive (Gen) is never distinguished from Dat in feminine words; the leftmost OblFem and GenPl attributes end in +r and hence result often in the same form as NomMasc (but weak attributes differ). It’s the only case where quantifiers (NUM), which are plural by definition, can have an ending, +r. GenStd has either an +s or an +n suffix, where the former is stronger than DirNeut (das, –eins vs. des, –eines). Genitive is mostly used for possessives and substantives in this case can be (strong) attributes to other SBST, either by replacing a left-hand DET (mostly proper nouns, i.e. names) or as a right-hand apposition.
Nota bene: While attributes combine gender and number, many nouns require a different inflection stem (i.e. suffix, umlaut or both) for plural inflection and they have an inherent gender which their singular attributes have to agree with congruently. FemSg substantives don’t inflect; GenStd and DatPl substantives inflect whenever possible. Some NomStd substantives (esp. those with +n in Gen) can or must drop a final -en, -n, or -e.