<<Case---a particular form of a nominal, pronominal or modifier that expresses a syntactical relationship between other morpho-phonic forms found in a sentence.>>
Wrong-"case" as used in grammar, is a particular form of nouns, pronouns or adjectives showing a syntactical relationship. The only part of this one that is, maybe, correct is that an adjective could be considered a modifier.
<<Adjective---a part of speech that modifies a substantive by means of suffixes such as -er and -est or by functioning syntactically in a predicative or attributive way.>>
Wrong-and adjective is used to describe (therefore, perhaps modify) a noun or pronoun. There are thousands of adjectives that do not use -er or -est, though "lamer" and "lamest" come to mind in the currect discussion.
<<Declension---a class of words belonging to one language with the same or a similar system of inflections.>>
Not sure about this one-you've got some of the right words, but they don't seem to be in the right order...a declension is a group of nouns, pronouns and adjectives with a particular inflection and used to indicate the case of the words. For instance, in Latin, the inflection of a word tells if it is a subject or direct object.
puella, serva, columba, pictura, terra, pupa, and bulla are all nominative and therefore would be subjects.
puellam, servam, columban, picturam, terram, pupam, and bullam are all in the accusative case and would indicate that the words are being used as direct objects.
Declension also indicates gender. All of the words in the lists above are in the First Declension in Latin and therefore are feminine.
The following lists of words are in the Second Declension and are therefore masculine.
architectus, humanus, publicus, fungus
<<Gender---a grammatical category used to classify nouns, pronouns, adjectives and sometimes verbs. Grammatical gender is not necessarily associated with actual gender.>>
Just like the use of big words is not necessarily associated with intelligence.
Darn it, I know I've said not bait this "rational substantive", but it IS kinda fun....
Jewel-off to conjugate a few verbs