Notice the yellow boxes in the neuter paradigm. Only these three forms differ from the masculine paradigm. Don't make it too complicated! Remember, for this type of noun, the genitive ends in -ī and the neuter has three forms which differ from the masculine.
Masculine in -er
Then there are a few masculine nouns ending in -er in the nominative singular. For these nouns, sometimes the -e- remains in the stem throughout. For Example, in puer, boy, the stem is puer-
And sometimes the -e- drops from the genitive singular and all remaining forms. For Example, in ager, field, the stem is agr-
Now, for high school Latin, these boxes represent all you need to know about second declension nouns. However, to be thorough, a few such nouns are feminine. Place names, such as Corinthus, Samus, and Aegyptus are feminine. Most trees, such as fāgus (beech) and pirus (pear) are feminine. Also feminine are Greek nouns such as atomus, dialectus, methodus, paragraphus, and periodus. And a few other words, such as humus (ground). Although these words are all feminine, they will follow the declension of servus above.
There are also a few neuter nouns which follow the masculine declension. These include pelagus (sea), virus (venom), and vulgus (the common crowd).