Latin Grammar - All about Second Declension Masculine nouns

Where first declension includes mostly feminine nouns with -a- at the end of their base, second declension includes mostly masculine and neuter nouns with -o- at the end of their base. The second declension has different forms from the first declension but the uses of the cases are the same. However, there’s another important distinction between first and second declension: the -o- at the end of the base in second declension is weak and it doesn’t show up as -o- as often as the -a- shows up in first declension.The second-declension -o- can appear as -u-, as in the nominative singular (-us) or the accusative singular (-um) or it can appear as -i- as in the genitive singular and nominative plural (-i) and the dative and ablative plural (-is).Here are the endings for second declension masculine, beginning with the singular. The nominative singular: -us or in some cases -er. We’ll discuss that in a second. The genitive singular: -i Dative: -o Accusative: -um Ablative: -o and an irregular vocative: -ěIn the plural the endings are: -i -orum -is -os -is. The vocative is regular here in the plural. It’s identical to the nominative -i.In forming a second-declension masculine noun, do the same thing you did in first declension. Take a noun that belongs to that declension, remove the -ī genitive singular ending from the genitive singular form to get the base, and then add the endings on.Here’s an example of that process with the word filius meaning “son.” The base is fili-. To that you add the endings: filius, filii, filio, filium, filio -- we’ll talk about the vocative in a second -- filii, filiorum, filiis, filios, filiis.Note the vocative singular of this word: The Romans didn’t like taking a base that ended with -i and putting a short -e after it. That would have produced *filie and that sounded disagreeable to the Romans so they left the -e off the vocative ending if there was an -i- at the end of the base. Thus the vocative of filius is fili.Now let’s address a minor peculiarity involving second declension - I mentioned we’d touch on this later… This is that “later.”. The Romans for some reason did not like to end second-declension masculine nouns with -rus. So, if a base ended in -r, like the base puer- which means “boy,” the Romans, instead of going *puerus in the nominative singular, simply went puer. But you can see from the rest of the forms of this word the real base is puer- because the word declines puer, pueri, puero and so on.