"Er" and "Ir" endings in Second Declension Masculine

o puer, qui omnia nomini debes 
You, boy, who owe everything to a name 
(Insult that Mark Antony said to Octavian, who later became Caesar Augustus)

To remember:

Some second-declension masculine nouns end in -er or -ir, instead of -us.   For now, we will just be learning three.    You can tell they are second declension because the genitive singular ends in "i". 

Examples:
Puer = boy
Ager = field
Vir = man

This chart shows that they are declined just like the normal masculine 2nd declension nouns.

Note:  The genitive singular of "ager" is "agri" and so this is the stem that is used for the rest of the declension.

After you look at the chart, go on to the next section