Continuing to talk about grammar (and still using wikipedia as a reference, for convenience’s sake)...
tldr: Declension, Inflection, Cases.
Declension: in linguistics, declension is the inflection (changing the form of a word) of nouns, pronouns, adjectives, and articles to indicate number (at least singular and plural), case (nominative or subjective, genitive or possessive, etc.), and/or gender. A declension is also a group of nouns that follow a particular pattern of inflection.
Inflected language: Fusional languages or inflected languages are a type of synthetic languages, distinguished from agglutinative languages by their tendency to use a single inflectional morpheme to denote multiple grammatical, syntactic, or semantic features. For example, the Spanish verb comer ("to eat") has the first-person singular preterite tense form comí ('I ate'); the single suffix -í represents both the features of first-person singular agreement and preterite tense, instead of having a separate affix for each feature.
An illustration of fusionality is the Latin word bonus ("good"). The ending -us denotes masculine gender, nominative case, and singular number. Changing any one of these features requires replacing the suffix -us with a different one. In the form bonum, the ending -um denotes masculine accusative singular, neuter accusative singular, or neuter nominative singular.
Cases: Case is a special grammatical category of a noun, pronoun, adjective, participle or numeral whose value reflects the grammatical function performed by that word in a phrase, clause, or sentence. In some languages, nouns, pronouns, adjectives, determiners, participles, prepositions, numerals, articles and their modifiers take different inflected forms depending on what case they are in.
English has largely lost its case system, although personal pronouns still have three cases that are simplified forms of the nominative, accusative and genitive cases: subjective case (I, you, he, she, it, we, they, who, whoever), objective case (me, you, him, her, it, us, them, whom, whomever) and possessive case (my, mine; your, yours; his; her, hers; its; our, ours; their, theirs; whose; whosever). Forms such as I, he and we are used for the subject ("I kicked the ball"), whereas forms such as me, him and us are used for the object ("John kicked me").
Languages such as Sanskrit, Ancient Greek, Latin, Armenian, Hungarian, Hindi, Tibetan, Czech, Slovak, Turkish, Tamil, Romanian, Russian, Polish, Serbo-Croatian, Estonian, Finnish, Icelandic, Belarusian, Ukrainian, Lithuanian, Basque, Esperanto and the majority of Caucasian languages have extensive case systems, with nouns, pronouns, adjectives, and determiners all inflecting (usually by means of different suffixes) to indicate their case. The number of cases differs between languages: German and Icelandic have four; Turkish, Latin and Russian each have at least six; Armenian, Czech, Polish, Serbian, Croatian, Ukrainian, and Lithuanian have seven; Sanskrit has eight; Estonian and Finnish have fifteen, Hungarian has eighteen and Tsez has sixty-four.
More formally, case has been defined as "a system of marking dependent nouns for the type of relationship they bear to their heads". Cases should be distinguished from thematic roles such as agent and patient. They are often closely related, and in languages such as Latin several thematic roles have an associated case, but cases are a morphological notion, whereas thematic roles are a semantic one. Languages having cases often exhibit free word order, because thematic roles are not required to be marked by position in the sentence.
