The category of case of nouns

Free morphemes

Many English words cannot be broken up into smaller meaningful parts, for example, "cat", "swim", "and", "the", "red" and "when". They contain only one morpheme, and since they can go out on their own in sentences, are called free morphemes.
Some free morphemes are function words with a syntactic or grammatical purpose:

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  "And" joins words and sentences.
  
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  "The" indicates a specific, known noun e.g. "a moon" could be anywhere in the universe, but "the moon" is the one that goes round the earth.
  
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  "When" is a conjunction or adverb about time.
  

We don't make up new function words like these, and it's quite hard to explain what a lot of these words mean. They really only make sense when used in sentences.
The rest of our words are content words with a semantic purpose. They carry the main meanings we are talking or writing about, such as "cat", "swim", "red" and "always".
We can and do make up new content words all the time, and if they catch on, dictionaries catalogue them and they become part of the language.

Bound morphemes

Many English words are made up of two or more meaningful parts, for example the words "catty", "swimming" and "reddest" all contain two bits of meaning. The "y", "ing" and "est" are bound morphemes, which need to be attached to another morpheme before they become a word, and can go out in sentences.
Some of our bound morphemes serve a syntactic purpose i.e. they inflect nouns, adjectives or verbs. Since English puts these only at the ends of words, not at word beginnings, these are called inflectional suffixes.
Two of these are added to the ends of nouns:

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  Plurals: show there's more than one of something. The usual plural morpheme is "s" or "es" e.g. cat-cats, dog-dogs, witch-witches. However, sometimes we form plurals in unusual ways, in words that come from Old English, Latin or French e.g. mouse-mice, child-children, radius-radii, phenomenon-phenomena, genus-genera, larva-larvae, bureau-bureaux etc. Click here for more examples.
  
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  The possessive: shows that something belongs to a noun, and is written as an apostrophe + "s" e.g. the frog's leg, the city's skyline, the cat's pyjamas. If we are talking about two frogs having four legs, we just add a possessive apostrophe after the plural "s" ("the frogs' legs).
  

Two inflectional suffixes are added to the ends of adjectives:

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