There are three important rules to remember in this chapter: 

• (1) There are three types of third declension i-stem nouns. 

• The first is parisyllabic in which the nominative singular (ending in - is/-es) and genitive singular have the same number of syllables. 

• The second is monosyllabic in which the nominative singular has one syllable and two consonants at the end of its base. And the third type of i-stem includes neuter nouns with nominative singular forms ending in -e, -al, or -ar. 

• (2) All i-stem nouns have an extra -i- in the genitive plural producing an ending -ium. Neuters also have -i in the ablative singular and -ia in the nominative and accusative plural. 

• (3) English “with” corresponds with three uses of the ablative: the ablative of means which requires no preposition in Latin, the ablative of manner which can use cum or no preposition, and the ablative of accompaniment which uses cum always. 

I-stem nouns are a sub-category of third-declension nouns. The differences between regular third-declension and i-stem third-declension nouns are relatively minor, as in most cases, they just an additional -i-. Only in one form does the -i- displace the original third-declension ending -e and replace it with an -i. Here is the regular formation of third declension masculine and feminine nouns.

      M/F    I-Stem

--- - es --- ---

-is - um    →           ---        - ium

-i - ibus --- ----em -es --- ---

-e -ibus --- ---

Neuter nouns exhibit a few more differences. Here is a chart reminding you about the regular formation of neuter third-declension nouns, and here are the changes that are effected when a third-declension neuter noun is i-stem. 

   Neuter         Neuter I-Stem

--- - a --- - ia

- is - um --- - ium

- i - ibus → --- ---

--- - a --- - ia

- e - ibus - i ---

You can see that not only is the genitive plural changed to -ium, but there is an -i replacing the -e in the ablative singular, and the nominative and accusative plurals are -ia. But, identifying third-declension i-stem neuter nouns is much easier than masculine and feminine ones. 

There are three nominative singular endings which identify whether a third-declension noun is i-stem or not. If the nominative singular ends in...

1. -e 

2. -al  

3. -ar 

Then the neuter noun is i-stem. It’s that simple. 

1. Now with that being done, let’s turn our attention to ablatives. The first of them is the ablative of means which uses no preposition and shows the tool or instrument used to perform some action, for instance, “with a sword, with a rake, with his hands.” Here are some examples in Latin: 

• labore, “by means of work” or “with labor”; 

• armis, literally “by means of arms,” meaning “with weapons”; 

• viā “by means of the road,” that is, “by using the path.” 

• 1. The second use of the ablative is the ablative of manner which most often uses the preposition cum. It shows how something happened ─ with honor, with speed, with humor ─ the prepositional phrase being equivalent to an adverb. So, for example: 

• cum ratione means “with reason,” that is, “reasonably”; 

• cum sapientiā means “with wisdom,” that is, “wisely”; 

• cum animis means “with courage,” that is, “courageously/bravely.” 

1. The third use of the ablative is the ablative of accompaniment which must always have cum. It shows who also participated in some activity. Thus it is best translated in English as “along or together with.” Here are three Latin examples of the ablative of accompaniment: 

• cum puellis, meaning “together with the girls”; 

• cum Cicerone, “with Cicero”; 

• cum isto malo, “with that bad man.” 

Besides their formation it’s often easy to tell the difference among these three uses of the ablative by looking at the type of noun used as the object of cum (if there is a cum). 

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