Ward Powers explains that declensions have to do with the grouping together of the various noun paradigms according to their "family resemblances" (Learn to Read the Greek New Testament, 28).

There are three declensions or noun families in Koine Greek.

That there are three declensions is actually a creation of grammarians for classification purposes, and declension has no bearing on the translation of a noun. A.T. Robertson reports that it is only since the seventeenth century that modern grammars have delineated three declensions: the older grammars had ten or more (Historical Reasearch, 246)!

In this module we will be studying second declension nouns. Traditionally, students have been introduced to the second declension first, because they are the most common nouns in the GNT, and because they may be learned more rapidly than the other two declensions.

• The first declension is characterized by endings with a-class vowels (i.e., a and h).
• The second declension is characterized by o-class vowels (i.e., o and w).
• The third declension is really a "grab bag" of nouns that cannot be categorized into the first or second declensions. However, it does consist of nouns with consonant stems.

In the second declension o-class vowels (i.e., o and w) predominate in the endings. Here are the second declension endings for masculine (sometimes feminine) and neuter nouns:

• Observe that the second declension neuter endings are identical to the masculine, except for those endings that are in gray.
• Observe also that the nominative, accusative, and vocative of neuter nouns are identical in both the singular and plural. Co-text must determine what case is being employed.
• The second declension masculine singular has a separate form for the vocative case. Otherwise, the nominative and vocative forms will be identical in all other instances, as in the neuter ending paradigm.
• The genitive plural form wn will be constant throughout all three declensions and in all three genders.
• Notice that the letter i is a hallmark of all dative endings (this will be true for all declensions). In both the first and second declension singular forms the iota is written in subscript. This subscript iota must not be omitted when spelling words.

What is a "paradigm"?

I have been freely using the word "paradigm." But what is a paradigm? The word paradigm is derived from the Greek word para,deigma, which referred to an example or model of something. In Greek grammar, a paradigm is an example of how words of a similar type are formed. Thus, the declension of a;nqrwpoj is paradigmatic of all other masculine nouns in the second declension. 

(1) Gender. There are three genders in Greek: masculine, feminine, and neuter. Gender does not usually have anything to do with sexuality; but rather, together with the stated declension (and in many instances, the kind of noun stem), it helps to indicate what endings will be attached to the stem of a noun. You will ultimately be able to determine the gender of a noun by observing the definite article that follows it in its lexical entry.

Therefore, a;nqrwpoj( o` is a masculine noun; grafh,( h`, feminine; and te,knon( to,, neuter.

IT IS ABSOLUTELY CRITICAL THAT THE STUDENT MEMORIZE THE DEFINITE ARTICLE ALONG WITH THE NOUN AS IT APPEARS IN A LEXICON OR VOCABULARY LIST.

(2) Number. A given substantive can be either singular or plural in number. Verbs will usually agree with their subject in number:

dou/loi ble,pousi
slaves see

The verb ble,pousi agrees in number with its subject dou/loi, which is plural.

(3) Case. In English there are three cases: subjective, possessive, and objective. English nouns usually do not change their form to signify case (exceptions: personal pronouns, e.g., he [subjective], his [possessive], and him [objective]; as well as the 's for possessives), but are identified by a word's position within a clause. In Greek, case is indicated by:

• various endings which are affixed to the stem of a substantive

• co-text