Third declension

The Third declension is the catch all declension. It comprises nouns that do not end with α or η (First declension) or with ο (Second declension). A number of features make the third declension challenging:
 * There's a fair spread of genders with no one being particularly dominant.
 * Masculine and feminine nouns share (roughly) a set of endings while the neuters have their own.
 * The nominative singulars seem almost always unique in appearance. They do not have a uniform ending and often the gender can't be discerned from the nominative singular.
 * Because many of the stems could end with consonants, they react with the ending in all various ways, causing all kinds of transformations.

Third declension tips

 * Learn the genitive--the root is most visible in the genitive (most times)
 * Genitive singular is usually -ος or some transform like -ως
 * Genitive plural is, still, always -ων
 * Datives always have an iota
 * Neuter
 * The nominative and accusative is always the same for each number
 * The nominative/accusative singular is usually the bare stem (except if it ends with τ - τ is dropped)
 * The nominative/accusative plural always ends with -α
 * Neuter genitives and datives still respect the rule that they conform to the masculine endings so all three genders are the same.

Third declension endings

 * mnemonic for masc./fem. : sosian esonsias
 * mnemonic for neuter : osi aonsia

Standard endings
By far one of the best behaved paradigms is actually from the adjective πᾶς. In the masculine and neuter forms, this adjective demonstrates all the standing endings.

This shows all the standard ends where the root was πάντ,except for the nominative and accusative neuter. The bare stem would have been πάντ but τ cannot stand at the end of a word, so it drops off.

Another example is neuter πνεῦμα where the root is πνεῦματ

Stems which end in –ι
This is a special case with the following features:
 * The unchanged stem is only visible in the nominative and accusative. Elsewhere –ι is replaced by –ε.
 * So the genitive is no clue to the stem. The genitive singular ending changes –ος by lengthening to –ως.
 * Genitive plural is still –εων.
 * Accusative singular uses –ν, basically behaving like 1st or 2nd declension.
 * These nouns are typically feminine.

An example is πολις.

Also βρῶσις.

Stems ending with -ευ
These words include βασιλεύς (king) and quite a number of "occupations". Note that the –υ  hardly stays and usually drops or changes